All emails duplicated

[#1]

For a couple of weeks now, all my emails have been duplicated.  This is happening on both Thunderbird on my computer and the email app on my phone (connecting using imap), and to both my main mailbox and the mailboxes used by others of my family on their devices.  I can see them on the webmail interface as well.

I've looked at the full headers of a couple of pairs and cannot see any difference. 

[#2]

Hi, A warm welcome to the forums.

Is this just with old emails or are new ones duplicated as well?

When did this happen - can you associate any activity on any of the connected clients with this occurring?

What clients are connected to this mailbox?

I think I have seen similar before, but I cannot clearly recall the circumstances, mail service etc ... but I could be thinking of calendar and contact details with iPhones with exchange.  Answers to be above might clarify the context, but it seems like a sync problem.

Passing thought - have you had your mailbox restored?

[#3]

Thanks for helping.   To try to answer your questions:

It's only new emails on arrival.  Quite often one copy arrives and then the second a few minutes later (probably the next time the client polls).  They only duplicate - I never get three or more.

I think it started happening after I did a major tidy up and moved a lot of emails to local archives  (which I've found in the past can confuse things for a while locally).

As you say, something seems to be out of sync, but it's across multiple clients on multiple machines.  

And no, I've not had anything restored.

[#4]

POP3 or IMAP email client?

If POP3 do you have more than one email address associated with the POP3 configuration?

[#5]

Everything is imap.  I didn't change the configuration on anything before it happened.

I've got multiple mailboxes, and some clients only connect to one of these mailboxes (I split emails to a family domain to separate mailboxes, and individuals connect to these mailboxes).  They all started to get duplicates at the same time.  So whatever's happening doesn't seem to be a specific client getting confused.