diff --git a/.github/copilot-instructions.md b/.github/copilot-instructions.md
index 94bf5e675..7bd3b0b44 100644
--- a/.github/copilot-instructions.md
+++ b/.github/copilot-instructions.md
@@ -63,7 +63,7 @@ When updating package versions:
 
 Before submitting:
 1. Test concretization with builder stage builds
-2. Verify no unexpected duplicate packages
+2. Verify no unexpected duplicate packages beyond each environment's `concretizer.duplicates.max_dupes` settings
 3. **Check BOTH CI workflows pass** (GitHub Actions AND GitLab/EICweb)
 4. Update documentation if architecture changes
 
diff --git a/.github/workflows/build-push.yml b/.github/workflows/build-push.yml
index 093caee8b..d616b86fe 100644
--- a/.github/workflows/build-push.yml
+++ b/.github/workflows/build-push.yml
@@ -369,7 +369,6 @@ jobs:
           runner: ubuntu-latest
           PLATFORM: linux/amd64
           target: final
-          SPACK_DUPLICATE_ALLOWLIST: "epic|llvm|py-setuptools|py-urllib3"
         - BUILD_IMAGE: eic_
           BUILD_TYPE: default
           BUILDER_IMAGE: debian_stable_base
@@ -379,7 +378,6 @@ jobs:
           runner: ubuntu-latest
           PLATFORM: linux/amd64/v3
           target: final
-          SPACK_DUPLICATE_ALLOWLIST: "epic|llvm|py-setuptools|py-urllib3"
         - BUILD_IMAGE: eic_
           BUILD_TYPE: default
           BUILDER_IMAGE: debian_stable_base
@@ -389,7 +387,6 @@ jobs:
           runner: ubuntu-24.04-arm
           PLATFORM: linux/arm64
           target: final
-          SPACK_DUPLICATE_ALLOWLIST: "epic|llvm|py-setuptools|py-urllib3"
         - BUILD_IMAGE: eic_
           BUILD_TYPE: default
           BUILDER_IMAGE: debian_stable_base
@@ -399,7 +396,6 @@ jobs:
           runner: ubuntu-latest
           PLATFORM: linux/amd64
           target: final
-          SPACK_DUPLICATE_ALLOWLIST: "epic|llvm|py-setuptools|py-urllib3|py-dask|py-dask-awkward|py-dask-histogram|py-distributed|py-requests"
         - BUILD_IMAGE: eic_
           BUILD_TYPE: default
           BUILDER_IMAGE: debian_stable_base
@@ -409,7 +405,6 @@ jobs:
           runner: ubuntu-latest
           PLATFORM: linux/amd64/v3
           target: final
-          SPACK_DUPLICATE_ALLOWLIST: "epic|llvm|py-setuptools|py-urllib3|py-dask|py-dask-awkward|py-dask-histogram|py-distributed|py-requests"
         - BUILD_IMAGE: eic_
           BUILD_TYPE: default
           BUILDER_IMAGE: debian_stable_base
@@ -419,7 +414,6 @@ jobs:
           runner: ubuntu-24.04-arm
           PLATFORM: linux/arm64
           target: final
-          SPACK_DUPLICATE_ALLOWLIST: "epic|llvm|py-setuptools|py-urllib3|py-dask|py-dask-awkward|py-dask-histogram|py-distributed|py-requests"
         - BUILD_IMAGE: eic_
           BUILD_TYPE: default
           BUILDER_IMAGE: cuda_devel
@@ -429,7 +423,6 @@ jobs:
           runner: ubuntu-latest
           PLATFORM: linux/amd64
           target: builder_concretization_default
-          SPACK_DUPLICATE_ALLOWLIST: "epic|llvm|py-setuptools|py-urllib3"
         - BUILD_IMAGE: eic_
           BUILD_TYPE: default
           BUILDER_IMAGE: cuda_devel
@@ -439,7 +432,6 @@ jobs:
           runner: ubuntu-latest
           PLATFORM: linux/amd64
           target: builder_concretization_default
-          SPACK_DUPLICATE_ALLOWLIST: "epic|llvm|py-setuptools|py-urllib3|py-dask|py-dask-awkward|py-dask-histogram|py-distributed|py-requests"
       fail-fast: false
     steps:
       - name: Free Disk Space (Ubuntu)
@@ -559,7 +551,6 @@ jobs:
             RUNTIME_IMAGE=${{ matrix.RUNTIME_IMAGE }}
             INTERNAL_TAG=${{ env.INTERNAL_TAG }}
             ENV=${{ matrix.ENV }}
-            SPACK_DUPLICATE_ALLOWLIST=${{ matrix.SPACK_DUPLICATE_ALLOWLIST }}
           cache-from: |
             type=registry,ref=${{ env.GH_REGISTRY }}/${{ env.GH_REGISTRY_USER }}/buildcache:${{ matrix.BUILD_IMAGE }}${{ matrix.ENV }}-${{ matrix.BUILD_TYPE }}-${{ env.GITHUB_REF_POINT_SLUG }}-${{ matrix.arch }}
             type=registry,ref=${{ env.GH_REGISTRY }}/${{ env.GH_REGISTRY_USER }}/buildcache:${{ matrix.BUILD_IMAGE }}${{ matrix.ENV }}-${{ matrix.BUILD_TYPE }}-${{ env.GITHUB_BASE_REF_SLUG }}-${{ matrix.arch }}
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index e86ec08c2..71e9993c0 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -479,16 +479,6 @@ eic:
                      --build-arg JUGGLER_SHA=$(sh .ci/resolve_git_ref eic/juggler ${JUGGLER_VERSION:-main})
                    }
                    --build-arg ENV=${ENV}
-                   --build-arg SPACK_DUPLICATE_ALLOWLIST=$(
-                     case "${ENV}" in
-                       ci|ci_without_acts|cuda|dbg|jl|prod)
-                         echo "epic|llvm|py-setuptools|py-urllib3" ;;
-                       xl|tf)
-                         echo "epic|llvm|py-setuptools|py-urllib3|py-dask|py-dask-awkward|py-dask-histogram|py-distributed|py-requests" ;;
-                       *)
-                         echo "epic|llvm|py-setuptools|py-urllib3" ;;
-                     esac
-                   )
                    --build-arg jobs=${JOBS}
                    --build-context spack-environment=spack-environment
                    --secret id=mirrors,src=mirrors.yaml
diff --git a/AGENTS.md b/AGENTS.md
index badfe9df3..e8365e2c3 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -214,7 +214,7 @@ The GitLab CI configuration (`.gitlab-ci.yml`) runs similar build jobs but in th
 
 When modifying `spack-environment/packages.yaml`:
 1. Test concretization: Build `builder_concretization_default` stage
-2. Check for duplicate packages (intentionally allowed: `epic`, `llvm`, `py-setuptools`, `py-urllib3`)
+2. Check for duplicate packages against the environment's `concretizer.duplicates.max_dupes` settings in `spack-environment/*/spack.yaml`
 3. Verify no unexpected duplicates appear in concretization output
 
 ### Build Testing
diff --git a/containers/eic/Dockerfile b/containers/eic/Dockerfile
index ccb2a59f3..ed22cc384 100644
--- a/containers/eic/Dockerfile
+++ b/containers/eic/Dockerfile
@@ -46,7 +46,6 @@ ARG ENV=xl
 ENV SPACK_ENV=/opt/spack-environment/${ENV}
 ARG SPACK_FLAGS="--backtrace"
 ARG SPACK_INSTALL_FLAGS="--no-check-signature --show-log-on-error --yes-to-all"
-ARG SPACK_DUPLICATE_ALLOWLIST=""
 ENV SPACK_COLOR="always"
 ENV GIT_TERMINAL_PROMPT=0
 
@@ -55,29 +54,6 @@ RUN <<EOF
 set -e
 spack env activate --without-view --dir ${SPACK_ENV}
 spack concretize --force
-spack --color=never find --long --no-groups --show-concretized --format "{name}" \
-| uniq -D -f2 \
-| tee /tmp/duplicates_all.txt
-if [ -n "${SPACK_DUPLICATE_ALLOWLIST}" ] ; then
-  grep -Evw "(${SPACK_DUPLICATE_ALLOWLIST})" /tmp/duplicates_all.txt \
-  | tee /tmp/duplicates_disallowed.txt
-else
-  cat /tmp/duplicates_all.txt \
-  | tee /tmp/duplicates_disallowed.txt
-fi
-if [ -s /tmp/duplicates_disallowed.txt ] ; then
-  echo "Duplicate packages found:"
-  cat /tmp/duplicates_disallowed.txt
-  cat /tmp/duplicates_disallowed.txt | while read status hash spec ; do
-    spack --backtrace spec --long /${hash}
-    if [ "$prevspec" = "$spec" ] ; then
-      spack diff /${hash} /${prevhash}
-    fi
-    prevhash=${hash}
-    prevspec=${spec}
-  done
-  exit 1
-fi
 EOF
 
 
@@ -198,29 +174,6 @@ if [ -n "${JUGGLER_SHA}" ] ; then
   spack deconcretize -y --all juggler
 fi
 spack concretize --force
-spack --color=never find --long --no-groups --show-concretized --format "{name}" \
-| uniq -D -f2 \
-| tee /tmp/duplicates_all.txt
-if [ -n "${SPACK_DUPLICATE_ALLOWLIST}" ] ; then
-  grep -Evw "(${SPACK_DUPLICATE_ALLOWLIST})" /tmp/duplicates_all.txt \
-  | tee /tmp/duplicates_disallowed.txt
-else
-  cat /tmp/duplicates_all.txt \
-  | tee /tmp/duplicates_disallowed.txt
-fi
-if [ -s /tmp/duplicates_disallowed.txt ] ; then
-  echo "Duplicate packages found:"
-  cat /tmp/duplicates_disallowed.txt
-  cat /tmp/duplicates_disallowed.txt | while read status hash spec ; do
-    spack --backtrace spec --long /${hash}
-    if [ "$prevspec" = "$spec" ] ; then
-      spack diff /${hash} /${prevhash}
-    fi
-    prevhash=${hash}
-    prevspec=${spec}
-  done
-  exit 1
-fi
 EOF
 
 
diff --git a/docs/spack-environment.md b/docs/spack-environment.md
index c21134782..88563648d 100644
--- a/docs/spack-environment.md
+++ b/docs/spack-environment.md
@@ -206,6 +206,17 @@ concretizer:
 - **`unify: true`**: Single version of each package in environment
 - **`reuse: false`**: Don't reuse packages from buildcache during concretization
 
+Environment-specific duplicate allowances are configured in each environment `spack.yaml`:
+
+```yaml
+spack:
+  concretizer:
+    duplicates:
+      max_dupes:
+        epic: 10
+        py-urllib3: 2
+```
+
 ## Key Packages
 
 ### Core Physics Packages
diff --git a/spack-environment/ci/spack.yaml b/spack-environment/ci/spack.yaml
index 4f01a2790..fb624b526 100644
--- a/spack-environment/ci/spack.yaml
+++ b/spack-environment/ci/spack.yaml
@@ -6,7 +6,11 @@ spack:
   - ../packages_root_without_opengl.yaml
   - ../view.yaml
   concretizer:
-    unify: when_possible
+    duplicates:
+      max_dupes:
+        llvm: 2
+        py-setuptools: 2
+        py-urllib3: 2
   specs:
   - acts
   - actsvg
diff --git a/spack-environment/ci_without_acts/spack.yaml b/spack-environment/ci_without_acts/spack.yaml
index 8148f3c24..daae356c1 100644
--- a/spack-environment/ci_without_acts/spack.yaml
+++ b/spack-environment/ci_without_acts/spack.yaml
@@ -6,7 +6,11 @@ spack:
   - ../packages_root_without_opengl.yaml
   - ../view.yaml
   concretizer:
-    unify: when_possible
+    duplicates:
+      max_dupes:
+        llvm: 2
+        py-setuptools: 2
+        py-urllib3: 2
   specs:
   - actsvg
   - afterburner
diff --git a/spack-environment/concretizer.yaml b/spack-environment/concretizer.yaml
index 80503f149..ae43ae8d9 100644
--- a/spack-environment/concretizer.yaml
+++ b/spack-environment/concretizer.yaml
@@ -2,9 +2,8 @@
 # - We disable reuse of build cache products during concretization
 #   (but build cache products will get used if available during install).
 # - We want unified environments that can be installed in a simple view,
-#   but unfortunately that prevents multiple geometry versions. We use
-#   when_possible in those containers where multiple geometry versions are
-#   installed, and consider other duplicate packages an error.
+#   with package-specific duplicate allowances configured per environment
+#   through concretizer.duplicates.max_dupes in each spack.yaml.
 #
 concretizer:
   reuse:
diff --git a/spack-environment/cuda/epic/spack.yaml b/spack-environment/cuda/epic/spack.yaml
index 44abdfdd4..7d8e24499 100644
--- a/spack-environment/cuda/epic/spack.yaml
+++ b/spack-environment/cuda/epic/spack.yaml
@@ -7,7 +7,12 @@ spack:
   include_concrete:
   - /opt/spack-environment/cuda
   concretizer:
-    unify: when_possible # multiple epic versions
+    duplicates:
+      max_dupes:
+        epic: 10
+        llvm: 2
+        py-setuptools: 2
+        py-urllib3: 2
   specs:
   - algorithms
   - edm4eic
diff --git a/spack-environment/cvmfs/spack.yaml b/spack-environment/cvmfs/spack.yaml
index 21e2de97d..b4014ec7e 100644
--- a/spack-environment/cvmfs/spack.yaml
+++ b/spack-environment/cvmfs/spack.yaml
@@ -7,7 +7,12 @@ spack:
   - ../packages_root_without_opengl.yaml
   - ../view.yaml
   concretizer:
-    unify: when_possible
+    duplicates:
+      max_dupes:
+        epic: 10
+        llvm: 2
+        py-setuptools: 2
+        py-urllib3: 2
   specs:
   - acts
   - actsvg
diff --git a/spack-environment/tf/epic/spack.yaml b/spack-environment/tf/epic/spack.yaml
index cdae8172f..c5ff5ac31 100644
--- a/spack-environment/tf/epic/spack.yaml
+++ b/spack-environment/tf/epic/spack.yaml
@@ -5,7 +5,16 @@ spack:
   - ../../packages.yaml
   - ../../view.yaml
   concretizer:
-    unify: when_possible  # included environment: py-numba needs a different llvm than the one provided as external
+    duplicates:
+      max_dupes:
+        llvm: 2
+        py-setuptools: 2
+        py-urllib3: 2
+        py-dask: 2
+        py-dask-awkward: 2
+        py-dask-histogram: 2
+        py-distributed: 2
+        py-requests: 2
   include_concrete:
   - /opt/spack-environment/tf
   specs:
diff --git a/spack-environment/tf/spack.yaml b/spack-environment/tf/spack.yaml
index b1beef89f..cb454d3a2 100644
--- a/spack-environment/tf/spack.yaml
+++ b/spack-environment/tf/spack.yaml
@@ -5,7 +5,16 @@ spack:
   - ../packages.yaml
   - ../view.yaml
   concretizer:
-    unify: when_possible  # py-numba needs a different llvm than the one provided as external
+    duplicates:
+      max_dupes:
+        llvm: 2
+        py-setuptools: 2
+        py-urllib3: 2
+        py-dask: 2
+        py-dask-awkward: 2
+        py-dask-histogram: 2
+        py-distributed: 2
+        py-requests: 2
   specs:
   - edm4hep
   - onnx
diff --git a/spack-environment/xl/epic/spack.yaml b/spack-environment/xl/epic/spack.yaml
index d724da572..44cf1dc7b 100644
--- a/spack-environment/xl/epic/spack.yaml
+++ b/spack-environment/xl/epic/spack.yaml
@@ -7,7 +7,12 @@ spack:
   include_concrete:
   - /opt/spack-environment/xl
   concretizer:
-    unify: when_possible # multiple epic versions
+    duplicates:
+      max_dupes:
+        epic: 10
+        llvm: 2
+        py-setuptools: 2
+        py-urllib3: 2
   packages:
     geant4:
       require: