Prevent swapping two equal items

sa.hpp

@@ -65,7 +65,8 @@ class box {
 	auto swappable11(box &other, int capacity) -> bool {
 		long long choice1 = items.top();
 		long long choice2 = other.items.top();
-		return choice1 + other.fullness - choice2 <= capacity &&
+		return choice1 != choice2 &&
+		       choice1 + other.fullness - choice2 <= capacity &&
 		       choice2 + fullness - choice1 <= capacity;
 	}