BIDSPath.match() can return duplicates (and files that don't exist)

[larsoner]

See

https://github.com/mne-tools/mne-bids/pull/1560/changes#diff-fa823e8c258bfc63ca666a1954e56b00ceb6816e16fecef3ba6cbffaa33de9b0R326

Where it returns hundreds of values when there are only 36 unique ones. This is due to finding paths like

a/b/something
a/c/something

in the initial scan and then replacing them with

bids_root/something
bids_root/something

I am not sure the correct fix here... deduplication is possible, but it's weird because the a/b and a/c are being replaced with bids_root so the bids_root/something file might not actually even exist in the end!