Fix a cause of Duplicate key error from MustFightBattle. (#12571)

* Fix a cause of Duplicate key error from MustFightBattle.

* Use a LinkedHashSet to preserve order, in case something depends on it.

game-app/game-core/src/main/java/games/strategy/triplea/delegate/battle/MustFightBattle.java

@@ -68,6 +68,7 @@
 import java.util.Comparator;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Optional;
@@ -571,8 +572,11 @@ public void removeUnits(
     if (killedUnits.isEmpty()) {
       return;
     }
+    // Note: Ideally we wouldn't have duplicates already, but this should fix the error for now.
+    // See: https://github.com/triplea-game/triplea/issues/11597
+    final var uniqueKilledUnits = new LinkedHashSet<>(killedUnits);
     final RemoveUnitsHistoryChange removeUnitsHistoryChange =
-        HistoryChangeFactory.removeUnitsFromTerritory(battleSite, killedUnits);
+        HistoryChangeFactory.removeUnitsFromTerritory(battleSite, uniqueKilledUnits);
 
     final Collection<Unit> killedUnitsIncludingDependents = removeUnitsHistoryChange.getOldUnits();
     final Collection<Unit> transformedUnits = removeUnitsHistoryChange.getNewUnits();