Show warning if about to add duplicate game

[jazzsequence]

May need to use autofill js on game title
If about to add a duplicate game, show a warning that says "did you already add this game?"

[jazzsequence]

A workaround exists which is just to search for the game before adding a new game. Still, this is a nice-to-have and could be easily built into the "Add Game From BGG" workflow immediately prior to creating the draft post.