Ignore invalid duplicate MarkMonitor domains

BCornwallrequested to merge
T393734-ignore-invalid-duplicate-domains into main

General purchase orders will populate in the MarkMonitor domain list and can incorrectly trap ncmonitor into thinking that they're invalid duplicates. We should just discard invalid registration statuses and only bail if there is somehow duplicate domains with valid registration statuses using a simple assertion check for the practically-zero chance that ever happens.

This will now only report for the "impossible" scenario of multiple valid MarkMonitor domain registrations. It will now happily service domains that were previously ignored because of extra invalid registrations.

As a bonus, this also reformats a little of the logic for get_domains() so its intention is clearer.

Bug: T393734

Closes T393734

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