diff --git a/src/objmem.cpp b/src/objmem.cpp
index e8b20e5b199..6d5d6879331 100644
--- a/src/objmem.cpp
+++ b/src/objmem.cpp
@@ -645,11 +645,27 @@ bool createFlagPosition(FLAG_POSITION **ppsNew, UDWORD player)
 	return true;
 }
 
+static bool isFlagPositionInList(FLAG_POSITION *psFlagPosToAdd)
+{
+	ASSERT_OR_RETURN(false, psFlagPosToAdd != nullptr, "Invalid FlagPosition pointer");
+	ASSERT_OR_RETURN(false, psFlagPosToAdd->player < MAX_PLAYERS, "Invalid FlagPosition player: %u", psFlagPosToAdd->player);
+	for (FLAG_POSITION* psCurr = apsFlagPosLists[psFlagPosToAdd->player]; (psCurr != nullptr); psCurr = psCurr->psNext)
+	{
+		if (psCurr == psFlagPosToAdd)
+		{
+			return true;
+		}
+	}
+	return false;
+}
+
 /* add the Flag Position to the Flag Position Lists */
 void addFlagPosition(FLAG_POSITION *psFlagPosToAdd)
 {
 	ASSERT_OR_RETURN(, psFlagPosToAdd != nullptr, "Invalid FlagPosition pointer");
 	ASSERT_OR_RETURN(, psFlagPosToAdd->coords.x != ~0, "flag has invalid position");
+	ASSERT_OR_RETURN(, psFlagPosToAdd->player < MAX_PLAYERS, "Invalid FlagPosition player: %u", psFlagPosToAdd->player);
+	ASSERT_OR_RETURN(, !isFlagPositionInList(psFlagPosToAdd), "FlagPosition is already in the list!");
 
 	psFlagPosToAdd->psNext = apsFlagPosLists[psFlagPosToAdd->player];
 	apsFlagPosLists[psFlagPosToAdd->player] = psFlagPosToAdd;