Is perhaps the laptop the only one configured to use IMAP mail instead of POP?

Shevyzn said

This has just started at the end of may and only on my lap top DELL Inspiron 15 3000 series. I use Thunderbird on other computors and they are fine.

Thanks Matt I have checked the POP settings and to me all is set to POP, the odd thing know is that my wifes laptop has started to do the same thing know.

I am at the point of deleting Thunderbird as I cannot get help with this problem of mails duplicating them-self continually.