[BUG] Mapping disappear when player gains nationality

[p1234091]

When generating new player's faces (on Preserve mode), if a newgen that already has a face gains a second nationality, he loses that mapping.

To Reproduce
Steps to reproduce the behavior:

1. Get .rtf of newgens from current save and generate mapping
2. Reload skin in FM, newgens should now have new faces
3. Modify the .rtf file: specifically, add a second nationality on a random newgen P in the .rtf
4. Re-generate mapping using Preserve mode, duplicates allowed (I didn't try other modes, can't speak about them) and reload skin in FM
5. The newgen P with second nationality loses his original face (he now doesn't have any face)

Expected behavior
Should be no-op for newgens that already have faces even with nationality change (UID is the same).

In repro steps above, if after step 4, we:
5. Remove second nationality of P in the .rtf
6. Re-generate the mapping using Preserve, duplicates allowed

Then P's face is back to what it was, so this seems like a potential error with reading/writing the mapping.