Dedupe unknown query params from feature flag query params.

[bmd3k]

Since #6784 we have seen the unusual behavior where query params for feature flags will be duplicated on the command line.

For example, if we load localhost:6006/?showFlags= it will be rewritten as localhost:6006/?showFlags=&showFlags=.

This change fixes this by deduplicating "unknown" flags with known feature flag query params.