The duplicates you get with a download usually will go away if you give them enough time to actually complete the download of the entire message. I have found a couple of "newsletter" sites that seem to keep sending repeats of the same things, but I suspect it's because they may have me on two different spam lists. In a case like this, the "sent" timestamp will at least be different on the two (or more) copies.

The delay in AV is the most common cause of dupes at the time of receiving mail, and usually one copy will "disappear" if you give it long enough. It's supposedly possible that the same message, delayed on one routing to you, might catch up with a copy that was re-sent via another route due to the delay on the first try.

Because email is normally kept in a database format, it is entirely possible to have the identically same message, as "permanent" copies, more than once. This should happen rarely with normal mail receiving, but it can be a real problem if you import a backup email store that contains copies of stuff still in your regular email folders. You can end up with many many duplicates that can be really tough to separate.

John