sync causes duplicate emails

only set up emclient  yesterday and it downloaded the 2000 plus emails from the server, this morning it did a sync and downloaded them again! deduplicator did not work! How do I prevent this happening again?

That is strange because the deduplicator is usually quite good with detecting duplicates. It obviously does not see them as identical.

Are you using IMAP or POP3?

POP 3 (the same issue for me )