All messages in my inbox on the Zoho server are being downloaded as many as 7 times with POP to my email client (Thunderbird). It is one copy of every existing (new and old), inbox message when the email client requests new messages, repeated each request time. 

The problem first occurred several weeks ago, when it lasted for several days, then cleared up with no action from me (presumably Zoho fixed the problem). It started again yesterday. 

My wife and I both use Zoho email accounts on different computers, both with Thunderbird as the POP email client. There were no issues on either computer prior to the several day incident a few weeks ago. The problem started on both computers at the same time and ended at the same time. There were no problems in the intervening several weeks and then the problem started again on both computers yesterday. On both computers, Thunderbird is configured with other POP mail accounts, but no other account has had a problem. 

During the episode several weeks ago, my Zoho account had probably a few hundred messages in the inbox, so there were many hundreds of bogus emails to clean up. My wife's account had a few thousand messages, so she had many thousands of bogus messages to clean up. We used webmail to access the accounts and move/delete the old inbox messages to minimize the problem. This episode, there are hundreds of bogus messages requiring manual cleanup again.

The pattern (all messages, including old ones that previously were not resent, occurring on multiple computers at exactly the same time and reliably repeating for several days, previously clearing up with no action on our end, and happening only with Zoho), suggests that the problem is not with Thunderbird or a bad Internet connection.

My assumption is that Zoho flags messages on its server that are downloaded, since many would no longer be in the client inbox at the time of the request for new messages. Since old messages have not previously been continuously re-downloaded, this suggests that, at least until the problem, the previously downloaded messages were properly flagged.  The problem appears to lie with the Zoho server or the download flags having been corrupted.