Correctly check for duplicate input parameters

[aschempp]

This fixes issue when manually matching requests, e.g. for the search indexer.

The mentioned issue (contao/core#4277) describes multiple cases, and it is where we added the unused argument stuff. These lines were previously added to prevent duplicate parameters in the URL.

1. For http://example.com/foo/bar/foo/baz.html, Contao (previously) correctly rendered the page with foo being set to baz. This also allowed for duplicate page URLs (e.g. http://example.com/foo/bar/foo/bar/foo/bar/foo/baz.html and the unused argument handling wouldn't kick in.
2. It should also not be possible to repeat a parameter that is already present in query parameters (e.g. http://example.com/foo/bar.html?foo=baz).

In Contao 3, this meant checking the $_GET array for duplicate data. In Symfony, this should only match on the current request.

Warning: what this PR cannot solve (and wasn't solved before) is actually running multiple requests that change the $_GET parameters 🤷