Fix interact using sometimes having target==used (#6847)

Content.Shared/Interaction/IInteractUsing.cs

@@ -4,6 +4,7 @@ using JetBrains.Annotations;
 using Robust.Shared.Analyzers;
 using Robust.Shared.GameObjects;
 using Robust.Shared.Map;
+using Robust.Shared.Utility;
 
 namespace Content.Shared.Interaction
 {
@@ -78,6 +79,11 @@ namespace Content.Shared.Interaction
 
         public InteractUsingEvent(EntityUid user, EntityUid used, EntityUid target, EntityCoordinates clickLocation, bool predicted = false)
         {
+            // Interact using should not have the same used and target.
+            // That should be a use-in-hand event instead.
+            // If this is not the case, can lead to bugs (e.g., attempting to merge a item stack into itself).
+            DebugTools.Assert(used != target);
+
             User = user;
             Used = used;
             Target = target;