Mail problem

Since upgrading (G5Dual) to Panther ,Mail is doing some weird stuff. I keep getting some emails more than once that I know have been only sent once - I've got account preferences set to delete files from the ISP's server once I've downloaded them. Also this deletion seems a bit erratic because I can go to account prefrences after Mail tells me it has removed all the files from the server and sometimes if I tell it to delete the files on the server straight away, it deletes a whole load of stuff that shouldn't be there as Masil has toldm,e it's already removed them???????