Actually, the issue was in SNAP 6, to allow two flags with the same name (SNAP-1094).
However, in SNAP 7 occurs another issue which then leads to the error. This will be fixed in an update soon.
However, Sentinel-5p is not supported by SNAP in the first place.
It can only be read by the generic NetCDF reader.
If you want to work with S5p data I would suggest that you use http://www.stcorp.nl/beat/