lupineshadow: If you use an email provider, just mark the duplicate emails as spam.
Surely with the GOG email titles as they are and the emails filled with external images and nothing else it wouldn't take much to push the spam score over the threshold and into your spam folder.
I don't have a spam folder, everything sent to me comes to me as is, except I preview it first direct on my ISP server with nPopUK, and just delete anything I don't want or is suspicious. I guess I have a Saved folder in nPopUK, for those I might like to keep a record of, but that just keeps the header and text, nothing else.
If GOG doesn't eventually solve this issue, then I may just use nPopUK to delete any duplicates, or just delete them after downloading to Thunderbird. I would like it to be solved though, and I am a bit wary of making a presumption that what appears to be a duplicate, going by title only, is actually a duplicate. Sometimes an error is realized after sending an email, which is then corrected and the email sent again.