Interestingly, quite a few places that should treat query strings transparently make a lot of assumptions about their structure. We ran into that when picking a new CDN, some providers didn't handle repeat parameters (?a=1&a=2) correctly.
For anyone curious like I was, form-urlencoded and the URLSearchParam API says that params should not be deduplicated or reordered. "Get" will get the first value with the given name, and GetAll will get a list of all values
Incorrectly would be processing the query string and deduping keys. Correctly would be passing it through as-is, or at least only lightly processing it, like normalizing escaping or such.
