I managed to create 2 custom fields with the same name without receiving an alert.
To do this, you need:
- create a field (not shared in the library) in a 1st project
- create the same field (still not shared in the library) in a 2nd project
- and then edit one of the 2 fields and share it in the library.
It then becomes possible to add 2 fields with the same name in a project.
I noticed that if you launch an advanced search with a filter on the field, then you can select only one of the 2 fields even though they have the same name.