I noticed after today's patch I now get 2 mails from the exchange when an item is sold. One mail that can be deleted and one that can't. After deleting the one, the other mail disappears after closing the window and opening it again.
Before this, the only time I would receive 2 mails from the exchange is if I had an item returned that expired. After claiming the item on the character who had it posted, then deleting that mail, the other copy of mail would be gone after closing the window and reopening it.
The problem with double mail from the exchange has been around for months, since Cryptic decided to nerf the mail system and make a lot of changes to it. Basically it happened, I think, after LoR launched but some time before the Risa summer event.
Yeah. I first thought it was laggy but as monkeybone13 describes, its paired emails; one of which will delete straight away and the other that doesn't until the mail window is reopened.
You can cycle through and delete in one session by jumping over the next message in turn and then closing/opening the window at the end, to finally clear the remaining duplicate chaff.
You have to wonder what the state of the codebase is. As we get areas, that logically shouldn't be connected to the separate area being fixed, being broken every time there's a patch. And mail apparently seems to be connected to absolutely everything in game - change a light bulb on ESD and mail will break!
It will indeed become a major issue when your mailbox gets full (300 messages, I think) and since you can't delete messages, you can't receive new ones - including the messages returning your items from the exchange.