ignore duplicate param values in request processing

[brockallen]

This PR adds logic when processing request parameters that are exact duplicates to filter/ignore them.

Fixes: https://github.com/DuendeSoftware/Support/issues/124

[brockallen]

We used to do it this was back in IdentityServer4. But I noticed here and here that dup parameters should treated as an invalid_request error (of course, I guess except for the resource parameter).

[leastprivilege]

The RFC says

Parameters sent without a value MUST be treated as if they were omitted from the request. The authorization server MUST ignore unrecognized request parameters. Request and response parameters MUST NOT be included more than once.

https://datatracker.ietf.org/doc/html/rfc6749#section-3.1

[brockallen]

Will rework for 6.2 to validate against dups.