diff --git a/libs/pattern/tests/Spec/Pattern/ReconcileProperties.hs b/libs/pattern/tests/Spec/Pattern/ReconcileProperties.hs
index 661d761..a278d31 100644
--- a/libs/pattern/tests/Spec/Pattern/ReconcileProperties.hs
+++ b/libs/pattern/tests/Spec/Pattern/ReconcileProperties.hs
@@ -222,9 +222,13 @@ spec = do
               let ids = collectAllIdentities reconciled
               in length ids === length (Set.fromList ids)
 
-      -- Full Merge support with element deduplication is implemented in Phase 4
       it "each identity appears at most once after reconciliation (Merge)" $
-        pendingWith "Full Merge with element deduplication is Phase 4 (User Story 2)"
+        property $ \(pattern :: Pattern Subject) ->
+          case reconcile (Merge UnionElements defaultSubjectMergeStrategy) pattern of
+            Left _ -> property True  -- Error case doesn't apply
+            Right reconciled ->
+              let ids = collectAllIdentities reconciled
+              in length ids === length (Set.fromList ids)
 
     describe "Determinism Properties" $ do
       it "reconciling the same pattern twice gives identical results (LastWriteWins)" $
diff --git a/specs/031-pattern-reconciliation/spec.md b/specs/031-pattern-reconciliation/spec.md
index cee7a6f..7121669 100644
--- a/specs/031-pattern-reconciliation/spec.md
+++ b/specs/031-pattern-reconciliation/spec.md
@@ -2,7 +2,7 @@
 
 **Feature Branch**: `031-pattern-reconciliation`
 **Created**: 2026-01-23
-**Status**: Draft
+**Status**: Implemented (2026-02-12)
 **Input**: User description: "Pattern Subject reconciliation as described in proposals/pattern-reconciliation.md"
 
 ## Clarifications