diff --git a/Core/HLE/sceKernelMutex.cpp b/Core/HLE/sceKernelMutex.cpp
index a591e4ce6d..4469dc0601 100644
--- a/Core/HLE/sceKernelMutex.cpp
+++ b/Core/HLE/sceKernelMutex.cpp
@@ -456,7 +456,10 @@ int sceKernelLockMutex(SceUID id, int count, u32 timeoutPtr)
 		return error;
 	else
 	{
-		mutex->waitingThreads.push_back(__KernelGetCurThread());
+		SceUID threadID = __KernelGetCurThread();
+		// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+		if (std::find(mutex->waitingThreads.begin(), mutex->waitingThreads.end(), threadID) == mutex->waitingThreads.end())
+			mutex->waitingThreads.push_back(threadID);
 		__KernelWaitMutex(mutex, timeoutPtr);
 		__KernelWaitCurThread(WAITTYPE_MUTEX, id, count, timeoutPtr, false);
 
@@ -481,7 +484,10 @@ int sceKernelLockMutexCB(SceUID id, int count, u32 timeoutPtr)
 		return error;
 	else
 	{
-		mutex->waitingThreads.push_back(__KernelGetCurThread());
+		SceUID threadID = __KernelGetCurThread();
+		// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+		if (std::find(mutex->waitingThreads.begin(), mutex->waitingThreads.end(), threadID) == mutex->waitingThreads.end())
+			mutex->waitingThreads.push_back(threadID);
 		__KernelWaitMutex(mutex, timeoutPtr);
 		__KernelWaitCurThread(WAITTYPE_MUTEX, id, count, timeoutPtr, true);
 
@@ -813,7 +819,10 @@ int sceKernelLockLwMutex(u32 workareaPtr, int count, u32 timeoutPtr)
 		LwMutex *mutex = kernelObjects.Get<LwMutex>(workarea.uid, error);
 		if (mutex)
 		{
-			mutex->waitingThreads.push_back(__KernelGetCurThread());
+			SceUID threadID = __KernelGetCurThread();
+			// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+			if (std::find(mutex->waitingThreads.begin(), mutex->waitingThreads.end(), threadID) == mutex->waitingThreads.end())
+				mutex->waitingThreads.push_back(threadID);
 			__KernelWaitLwMutex(mutex, timeoutPtr);
 			__KernelWaitCurThread(WAITTYPE_LWMUTEX, workarea.uid, count, timeoutPtr, false);
 
@@ -846,7 +855,10 @@ int sceKernelLockLwMutexCB(u32 workareaPtr, int count, u32 timeoutPtr)
 		LwMutex *mutex = kernelObjects.Get<LwMutex>(workarea.uid, error);
 		if (mutex)
 		{
-			mutex->waitingThreads.push_back(__KernelGetCurThread());
+			SceUID threadID = __KernelGetCurThread();
+			// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+			if (std::find(mutex->waitingThreads.begin(), mutex->waitingThreads.end(), threadID) == mutex->waitingThreads.end())
+				mutex->waitingThreads.push_back(threadID);
 			__KernelWaitLwMutex(mutex, timeoutPtr);
 			__KernelWaitCurThread(WAITTYPE_LWMUTEX, workarea.uid, count, timeoutPtr, true);
 
diff --git a/Core/HLE/sceKernelSemaphore.cpp b/Core/HLE/sceKernelSemaphore.cpp
index f9e5cd4e0b..8fbeb4e674 100644
--- a/Core/HLE/sceKernelSemaphore.cpp
+++ b/Core/HLE/sceKernelSemaphore.cpp
@@ -360,7 +360,11 @@ int __KernelWaitSema(SceUID id, int wantedCount, u32 timeoutPtr, const char *bad
 		else
 		{
 			s->ns.numWaitThreads++;
-			s->waitingThreads.push_back(__KernelGetCurThread());
+
+			SceUID threadID = __KernelGetCurThread();
+			// May be in a tight loop timing out (where we don't remove from waitingThreads yet), don't want to add duplicates.
+			if (std::find(s->waitingThreads.begin(), s->waitingThreads.end(), threadID) == s->waitingThreads.end())
+				s->waitingThreads.push_back(threadID);
 			__KernelSetSemaTimeout(s, timeoutPtr);
 			__KernelWaitCurThread(WAITTYPE_SEMA, id, wantedCount, timeoutPtr, processCallbacks);
 		}