Add duplicate hotkey tracking with a boolean property on the definition

OpenRA.Game/HotkeyDefinition.cs

@@ -20,6 +20,7 @@ namespace OpenRA
 		public readonly Hotkey Default = Hotkey.Invalid;
 		public readonly string Description = "";
 		public readonly HashSet<string> Types = new HashSet<string>();
+		public bool HasDuplicates = false;
 
 		public HotkeyDefinition(string name, MiniYaml node)
 		{