Detect duplicate files

[tompollard]

A dataset submitted to PhysioNet appears to unintentionally include duplicate files:
https://physionet.org/content/cebsdb/1.0.0/

e.g. search for "3e0ccc9e8960427710b47cfbd2531a20fdb73c247bc9f21795e31fecf7069c8d" in https://physionet.org/content/cebsdb/1.0.0/SHA256SUMS.txt

It might be worth adding a duplicate-file detector to the validator.