Another cause for repeated emails is that the email client (OE) is not disconnecting from the server before it can delete the messages from the server.
If, for instance, your Internet connection drops out after all the emails have come in but before OE says "Completed", then the chances are it hasn't yet deleted the messages from the server. A second run of Send/Receive all will receive the same emails all over again.
Another one, more pertinent to repeated topic subscription notifications, is this:
Suppose you subscribe to a thread (here, you are automatically subscribed merely by replying).
The board software will send you one email to say the topic has been responded to .... if someone else replies before you come back to view the board, then it won't send another one.
But if you come on the board after a notification has been sent, but not checked your emails, then the board will 'think' you've read the thread in question. Should someone else then post into it, you'll receive a second email notifying you of a response.
Point to ponder: Check the date/time on the email saying when it has come in. If the date/times are different for a notification, then the chances are it's the second scenario. If the date/times are identical, then it's more likely to be the first.
Try also logging onto your emails via the the web portal, and seeing if there are multiple emails there as well. That may well help you confirm / deny the above.
Hope this helps!