diff --git a/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt b/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
index b4612ef5..15643434 100644
--- a/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
+++ b/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
@@ -45,6 +45,11 @@ class MainViewModel @Inject constructor(
     private val _hasEnteredSession = MutableStateFlow(false)
     val hasEnteredSession: StateFlow<Boolean> = _hasEnteredSession.asStateFlow()
 
+    // Synchronous flag to prevent participant_joined from adding duplicates after session_started
+    // This is set immediately (synchronously) when session_started is processed, ensuring
+    // any subsequent participant_joined events in the same callback queue will see the flag
+    private var sessionStartedForSession: String? = null
+
     init {
         // Start initialization on app launch
         checkAllServices()
@@ -339,6 +344,12 @@ class MainViewModel @Inject constructor(
 
             when (event.eventType) {
                 "session_started" -> {
+                    // CRITICAL: Set flag immediately (synchronously) to prevent subsequent
+                    // participant_joined events from adding duplicates. This must be the
+                    // first line before any async operations.
+                    sessionStartedForSession = event.sessionId
+                    android.util.Log.d("MainViewModel", "Session started flag set for: ${event.sessionId}")
+
                     // Check if this is for keygen initiator (CreateWallet)
                     val currentSessionId = _currentSessionId.value
                     if (currentSessionId != null && event.sessionId == currentSessionId) {
@@ -396,11 +407,12 @@ class MainViewModel @Inject constructor(
                 "party_joined", "participant_joined" -> {
                     android.util.Log.d("MainViewModel", "Processing participant_joined event...")
 
-                    // Don't add participants if keygen/sign has already started
-                    // This prevents duplicate additions after session_started event
-                    val currentStatus = repository.sessionStatus.value
-                    if (currentStatus == SessionStatus.IN_PROGRESS || currentStatus == SessionStatus.COMPLETED) {
-                        android.util.Log.d("MainViewModel", "  Session already in progress/completed, ignoring participant_joined")
+                    // CRITICAL: Check synchronous flag first - if session_started was already
+                    // processed for this session, don't add more participants
+                    // This is 100% reliable because the flag is set synchronously in session_started
+                    // handler before any async operations, and callbacks are processed sequentially
+                    if (sessionStartedForSession == event.sessionId) {
+                        android.util.Log.d("MainViewModel", "  Session already started for ${event.sessionId}, ignoring participant_joined")
                         return@setSessionEventCallback
                     }
 
@@ -548,6 +560,8 @@ class MainViewModel @Inject constructor(
         _publicKey.value = null
         _createdInviteCode.value = null
         _hasEnteredSession.value = false
+        // Reset synchronous flag for fresh session
+        sessionStartedForSession = null
         // Reset session status to WAITING for fresh start
         repository.resetSessionStatus()
     }
@@ -740,6 +754,8 @@ class MainViewModel @Inject constructor(
         pendingJoinToken = ""
         pendingPassword = ""
         pendingJoinKeygenInfo = null
+        // Reset synchronous flag for fresh session
+        sessionStartedForSession = null
         // Reset session status to WAITING for fresh start
         repository.resetSessionStatus()
     }
@@ -925,6 +941,8 @@ class MainViewModel @Inject constructor(
         pendingCoSignInviteCode = ""
         pendingCoSignJoinToken = ""
         pendingJoinSignInfo = null
+        // Reset synchronous flag for fresh session
+        sessionStartedForSession = null
         // Reset session status to WAITING for fresh start
         repository.resetSessionStatus()
     }
@@ -1671,6 +1689,8 @@ class MainViewModel @Inject constructor(
         _signature.value = null
         _txHash.value = null
         pendingSignInitiatorInfo = null
+        // Reset synchronous flag for fresh session
+        sessionStartedForSession = null
         // Reset session status to WAITING for fresh start
         repository.resetSessionStatus()
     }