Added duplicate rewrite entry check (partially fixes #4585)

[Bluscream]

Loop over existing entries and return an error if the entry already exists

[ainar-g]

Thank you, but I'd like to put this on hold, since the HTTP API and its code will be fully rewritten during the v0.108 cycle. Also, correct me if I'm wrong, but this won't check for duplicates in the data that is currently being sent?

[Bluscream]

This only accounts for sending one single rewrite because the current implementation doesn't work for multiple rewrites. 🤷🏻

[ainar-g]

Ah, you're right. I've been thinking ahead of myself :sweat_smile:.

[stale[bot]]

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