diff --git a/confidence-resolver/src/resolver_spec_tests.rs b/confidence-resolver/src/resolver_spec_tests.rs
index c2641103..f6dca833 100644
--- a/confidence-resolver/src/resolver_spec_tests.rs
+++ b/confidence-resolver/src/resolver_spec_tests.rs
@@ -120,6 +120,7 @@ struct SpecResolvedFlag {
     variant: String,
     value: Option<serde_json::Value>,
     should_apply: Option<bool>,
+    assignment_id: Option<String>,
 }
 
 // ---------------------------------------------------------------------------
@@ -424,6 +425,37 @@ fn run_spec_test(state: &ResolverState, test_case: &SpecTestCase) {
                 test_case.name, expected_flag.flag, expected_should_apply, actual_should_apply,
             );
         }
+
+        // assignmentId assertion (from resolve token)
+        if let Some(expected_assignment_id) = &expected_flag.assignment_id {
+            let token = resolver
+                .decrypt_resolve_token(&resolve_response.resolve_token)
+                .unwrap_or_else(|e| {
+                    panic!(
+                        "[{}] Failed to decrypt resolve token: {}",
+                        test_case.name, e,
+                    )
+                });
+            match token.resolve_token {
+                Some(flags_resolver::resolve_token::ResolveToken::TokenV1(token_v1)) => {
+                    let assigned = token_v1.assignments.get(&expected_flag.flag).unwrap_or_else(|| {
+                        panic!(
+                            "[{}] Flag '{}' not found in resolve token assignments",
+                            test_case.name, expected_flag.flag,
+                        )
+                    });
+                    assert_eq!(
+                        &assigned.assignment_id, expected_assignment_id,
+                        "[{}] assignmentId mismatch for flag '{}'",
+                        test_case.name, expected_flag.flag,
+                    );
+                }
+                _ => panic!(
+                    "[{}] Expected TokenV1 in resolve token",
+                    test_case.name,
+                ),
+            }
+        }
     }
 }
 
@@ -641,3 +673,6 @@ spec_test!(combined_both_match);
 spec_test!(combined_only_attr);
 spec_test!(combined_only_mat);
 spec_test!(combined_neither);
+
+// Materialization: duplicate variant assignments
+spec_test!(mat_same_variant_two_assignments_returns_correct_assignment_id);
diff --git a/confidence-resolver/test-payloads/resolver-spec/state.json b/confidence-resolver/test-payloads/resolver-spec/state.json
index c9c9519b..41b995c4 100644
--- a/confidence-resolver/test-payloads/resolver-spec/state.json
+++ b/confidence-resolver/test-payloads/resolver-spec/state.json
@@ -2031,6 +2031,41 @@
           }
         }
       ]
+    },
+    "flags/mat-same-variant-flag": {
+      "name": "flags/mat-same-variant-flag",
+      "state": "ACTIVE",
+      "clients": ["clients/test-client"],
+      "variants": [
+        { "name": "on", "value": { "enabled": true } }
+      ],
+      "rules": [
+        {
+          "name": "flags/mat-same-variant-flag/rules/rule1",
+          "segment": "segments/country-se",
+          "enabled": true,
+          "targetingKeySelector": "targeting_key",
+          "materializationSpec": {
+            "readMaterialization": "materializations/mat1",
+            "writeMaterialization": "materializations/mat1"
+          },
+          "assignmentSpec": {
+            "bucketCount": 1000,
+            "assignments": [
+              {
+                "assignmentId": "a1",
+                "variant": { "variant": "on" },
+                "bucketRanges": [{ "lower": 0, "upper": 500 }]
+              },
+              {
+                "assignmentId": "a2",
+                "variant": { "variant": "on" },
+                "bucketRanges": [{ "lower": 500, "upper": 1000 }]
+              }
+            ]
+          }
+        }
+      ]
     }
   },
   "segments": {
diff --git a/confidence-resolver/test-payloads/resolver-spec/tests.json b/confidence-resolver/test-payloads/resolver-spec/tests.json
index 57279dac..805aab60 100644
--- a/confidence-resolver/test-payloads/resolver-spec/tests.json
+++ b/confidence-resolver/test-payloads/resolver-spec/tests.json
@@ -3891,5 +3891,38 @@
         "expectError": null
       }
     }
+  },
+  {
+    "name": "mat_same_variant_two_assignments_returns_correct_assignment_id",
+    "resolveRequest": {
+      "flags": ["flags/mat-same-variant-flag"],
+      "evaluationContext": { "targeting_key": "user1", "country": "SE" },
+      "clientSecret": "test-secret"
+    },
+    "materializations": {
+      "user1": {
+        "materializations/mat1": {
+          "isUnitInMaterialization": true,
+          "ruleToVariant": {
+            "flags/mat-same-variant-flag/rules/rule1": "on"
+          }
+        }
+      }
+    },
+    "expectedResult": {
+      "general": {
+        "resolvedFlags": [
+          {
+            "flag": "flags/mat-same-variant-flag",
+            "reason": "RESOLVE_REASON_MATCH",
+            "variant": "on",
+            "value": { "enabled": true },
+            "shouldApply": true,
+            "assignmentId": "a2"
+          }
+        ],
+        "expectError": null
+      }
+    }
   }
 ]