When trying to merge two items with Special:MergeItems that are already merged, it outputs an error like: `Failed to load item. Error: Unresolved redirect to Q123.`. This is especially unhelpful if someone clicks on "Merge items" twice, thus they submit the form twice and the second merge attempt fails as the first one succeeds.
The special page should detect this case and output a more helpful error message, explaining that the item already is a redirect to that target.
We may also want to disable the form on the special page once it has been disabled, so that it can't be double-submitted (like suggested for Special:NewItem/ Special:NewProperty in T147143).