If I understand you correctly, when you view a message, if you scroll
down in the message window, the text is repeated.

Are the two copied identical or does the lower one have HTML tags in it
such a <B> <I>, <FONT...>?

If that's the case, you are receiving HTML format messages and the MIME
headers are corrupted so that OE doesn't display it correctly.  Most
likely this is caused by your anti-virus program scanning e-mail as it
downloads or an anti-spam program.