Not only did they take my money twice for renewing some domains (they sent me a notice after I'd renewed so to be safe and keep them, I renewed again and figured I'd sort it later) but they're also sending me SOMEONE ELSE'S renewal notices now!

Anyone else had problems like this with them?

Nope, it seems like I get renewal reminders weekly but they are always mine own domains

Just check if there is your email contact in the domains for which you are being sent the renewal notice...

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Like bagful said, run a whois and check on the administrative contact details. Are you sure you didn't sell this/these domains sometime in the past? The polite thing would be to try to contact the person by some other means in the whois (i.e.- phone / snail mail) so that they do not lose their domain.

Very strange,

One of the few problems i have ever read about at registerfly.