Adding the same folder twice leads to a buggy state

[Thomas Goldstein]

Right click on "Rom files", click "Add", and select a folder with ROMs.
Let's say the folder contains 3 valid ROMs.
Now just try to add that same folder again.

Result: you still only see a single folder on the left, but in it, you now see 6 files, 3 complete ones (green) and 3 incomplete ones (yellow).

[Eric Bole-Feysot]

Removing milestone: 4.0 (automated comment)