checksum-transcriber creates duplicate checksums

[stikonas]

We sometimes have the same file available from a few different sources. checksum-transcriber makes us check all those checksums multiple times:

 +> sha256sum -c sources.SHA256SUM 
/external/distfiles/mes-0.27.1.tar.gz: OK
/external/distfiles/nyacc-1.00.2-lb1.tar.gz: OK
/external/distfiles/nyacc-1.00.2-lb1.tar.gz: OK
/external/distfiles/nyacc-1.00.2-lb1.tar.gz: OK

$ cat sources.SHA256SUM 
183a40ea47ea49f8a1e3bd1b9d12e676374d64d63bc79e7bc1ae7d673dfdf25d  /external/distfiles/mes-0.27.1.tar.gz
708c943f89c972910e9544ee077771acbd0a2c0fc6d33496fe158264ddb65327  /external/distfiles/nyacc-1.00.2-lb1.tar.gz
708c943f89c972910e9544ee077771acbd0a2c0fc6d33496fe158264ddb65327  /external/distfiles/nyacc-1.00.2-lb1.tar.gz
708c943f89c972910e9544ee077771acbd0a2c0fc6d33496fe158264ddb65327  /external/distfiles/nyacc-1.00.2-lb1.tar.gz