Duplicate txn_id check doesn't filter out flagged IPNs

[rakiru]

If an IPN fails for some reason[1] and is flagged, any future IPN attempts will fail with a "Duplicate txn_id" error, even though we have received no previous successful IPN. I think all this needs is for duplicate_txn_id to filter out flagged IPNs before checking for previous ones, but I'd need to check since I'm not entirely familiar with the internals yet. I'll send a PR at some point if no-one else gets around to looking at it before I do.

[1] At least, if it fails while doing the verification postback
Mentioned in #117