Duplicated row ID when load mapping for GNPS 'fbmn' style files

[CunliangGeng]

Problem: the GNPSFileMappingLoader._load_mapping_fbmn overwrites the existing mapping for duplicated row ID.
Example file: tests/data/nodes_fbmn.csv file, row ID 1955 and 1957 are duplicated.

### Tasks
- [ ] Should the existing mapping be overwritten?
- [ ] if not, update the code

[CunliangGeng]

@hechth See the issue description please, should the existing mapping be overwritten?