moving Outlook data file with POP account

I changed computer and had to move my Outlook .pst data file containing my Yahoo POP messages. Everything is fine except for the very annoying fact that Outlook wants to download again the 3000+ emails I have in my Yahoo mailbox (they have been read already but stay on the serer).

Is there a way to avoid these duplicate downloads?
I don’t understand why Outlook can’t see that the .pst file already contains all these messages…

Thanks,
–laurent