There are certain buildings that only 1 of can be built in your empire, or only 1 of can be built in the world.

If you have the AI governor you can end up with multiple cities trying to build it. I can't tell if it just queued it up in some secondary city and I didn't notice and added it to my capital, or if the governor just added it to 2 different cities.

Either way this creates a problem since once one finishes, the other gets stuck at "unable to complete" and bugs you every turn until you manually cancel it (and don't get refunded)

My suggestion for that is that if any of your cities starts building a wonder, then the moment it is queued for construction in one city it would grey out in all other cities and no longer be possible to build there until it gets canceled in the first city. Hovering over a greyed out copy should inform you which city is currently building it,  how many turns are left before it finishes, and an estimate amount of time it would take for the current city to build it in case you change your mind and decide to cancel it in the original city and build in the other city instead

Incidentally, apparently "can only build one" is literal. That is, if you destroy the first one built, you cannot build a second one.