debuginfod: PR27983 - ignore duplicate urls

debuginfod: PR27983 - ignore duplicate urls

Gazing at server logs, one sees a minority of clients who appear to have
duplicate query traffic coming in: the same URL, milliseconds apart.
Chances are the user accidentally doubled her $DEBUGINFOD_URLS somehow,
and the client library is dutifully asking the servers TWICE. Bug #27863
reduces the pain on the servers' CPU, but dupe network traffic is still
being paid.  We should reject sending outright duplicate concurrent
traffic.
The urls are now simply removed upon finding a duplicate after url
construction.

https://sourceware.org/bugzilla/show_bug.cgi?id=27983

Signed-off-by: Noah Sanci <nsanci@redhat.com>