[Users] folder synchronization

[blind Pete]

Hi List, 

One of my e-mail accounts has a small quota so I decided to simply copy
the contents of its INBOX to a LocalMail folder.  So far, so good.
Some time later after receiving many messages and deleting many of low
importance I tried the same trick again, expecting that only the new
messages would be copied.  There was a great deal of duplication.  

Is there a sensible way to do this?  And to get rid of existing
duplicates?