Using Xp and Thunderbird.

Turned on my computer. Clicked on my email icon to get email which might have come in overnight and and got deluged with old email. It looks like every email which I had previously received during this month downloaded again. At last count, there were 730 emails that I had to delete again.

This has never happened before. I would prefer that it not happen again.

I did not do anything different this morning, before the problem. Turned the computer on. After it booted, I clicked on the email icon.

What is the problem? How do I prevent it from happening again?