Mailbox filling with duplicates in Sent Objects

I periodically have a problem with getting "Mailbox is almost full" messages. When this first happened, I could not figure out  why, since I was emptying the mailbox and kept getting the same fill level (1.90 out of 2  Gig, etc.). Then I discovered a folder called "Sent Objects" which was FULL of hundreds of duplicates to emails that were very old, but all had recent dates and over 1 MB in size. selected all and trashed them and the level finally went down. I told Cox about it and they had to refer me to a higher tech group who was supposed to get back to me, but never did.

That was a couple of months ago. But it has recently recurred, with the "Mailbox is 90% full" messages returning in the last few days. Sure enough, the Sent Objects folder was full of multiple copies of the same email. I again have deleted them over and over and they keep coming back.

Any help appreciated!