diff --git a/shared/tests/turn.characterization.test.js b/shared/tests/turn.characterization.test.js
index f3fc764..9def45b 100644
--- a/shared/tests/turn.characterization.test.js
+++ b/shared/tests/turn.characterization.test.js
@@ -338,9 +338,7 @@ describe('addParticipant', () => {
     expect(patch.participants.map(x => x.id)).toEqual(['a', 'z']);
   });
 
-  // SKIPPED: RED test documenting BUG-2 (addParticipant allows dup id).
-  // See TODO.md BUG-2. Re-enable (remove .skip) when fix lands.
-  test.skip('rejects duplicate id (skip-bug root cause)', () => {
+  test('rejects duplicate id (skip-bug root cause)', () => {
     // Two participants with same id → togglePause resume rebuilds order with
     // dup id twice → nextTurn gets stuck repeating that id forever.
     // Audit found this in 100-round replay (addParticipant fired while paused
diff --git a/shared/turn.js b/shared/turn.js
index bcdb897..4a9a0ae 100644
--- a/shared/turn.js
+++ b/shared/turn.js
@@ -270,6 +270,9 @@ function togglePause(encounter) {
 
 // ADD_PARTICIPANT — appends participant. (Initiative rolled by caller via build*.)
 function addParticipant(encounter, participant) {
+  if ((encounter.participants || []).some(p => p.id === participant.id)) {
+    throw new Error(`Participant with id "${participant.id}" already exists in encounter.`);
+  }
   const updatedParticipants = [...(encounter.participants || []), participant];
   return {
     patch: { participants: updatedParticipants },