When resending to single contact, entire mailing was resent!

Created by: artfulrobot

Do not use the "resend mailing to single contact" feature! [EDIT: if you're on 1.0.4+ it should be a bit safer, but still considered experiemental]

As the warning "this might do terrible things" suggests, turns out it can do just that.

I believe that the problem might be that if it crashes creating the entries for a job (which it seems to when you try to resend to someone who has already had that mailing), then it leaves a mailing job Scheduled, and I think that then the queue runner picks it up and executes it.

I think there's also the (more remote) possibility that a queue runner started async to the resend request sees the job in the milliseconds between creating the job and sending the single queue item.

A fix might be to wrap all the code in a trxn; to check for the contact given already being a recipient.