Hacked together to get around the AWS API Gateway issue documented here (Duplicated query string parameters are not supported).
All queries for a given prefix are expected to be a JSON array of strings. These are unmarshalled as individual strings, so they are accessible via url.Query().
For example, a url.URL of:
https://example.com?json_foo=["a","b","c"]
Becomes available from the url.URL:
HandleURL(url, "foo_")
foos := url.Query()["foo"]
fmt.Printf("%#v", foos) // []string{"a", "b", "c"}
You'll probably want to put this in http.Handler middleware.