mail triplicates downloading old messages

mail keeps downloading old messages.

occurs in two Mac Mini's one with 10.4.11 and other 10.6.8

Occurred in one of 7 mailboxes.

Any ideas what causes this.

(mail has be properly set to remove then after a week (as use 3 different computers from different desks, etc)

After 3rd down load of 86 messages finally had to remove them manually from that mailbox. worked immediately, but should not have been required

as normally it 'saw' previously messages and did not repeat the process.

This topic does not seem to have been visited since 2008