Make plugin able to override values only if they don't exist already

Currently there is no way to add query params to url only when a parameter with the same name already exist.
My case:
In my project "noredirect" param is always added to the url, but for some testcases I need to test redirects, so when I call `this.browser.url()` with an url containing `&noredirect=0`, it gets concatenated and I get `...&noredirect=0&noredirect=1`, which breaks the logic

The issue names no files, tests, or entry points; begin by locating the plugin's URL-decoration logic and its existing query-parameter handling. Done means an existing parameter is not duplicated when a later URL supplies the same name, while absent parameters retain current behavior; add focused coverage if the repository's tests are found.

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