Better check for valid items in the queue

Not an issue with this mod, but you can queue multiple copies of unique wonders, districts, and unrepeatable projects in the same city. This bug happens in vanilla (without mods). Another mod that changes the production panel fixed this bug, I figure CQUI could do so as well.

Start by inspecting CQUI's production panel and queue handling, then reproduce the vanilla behavior by queuing duplicate unique wonders, districts, or unrepeatable projects in one city. Done means the queue rejects invalid duplicate items while preserving valid production choices; compare the behavior with the other mod mentioned in the issue.

Written by the indexing model from the issue text.