Your email client my be using both TOP and RETR commands when receiving email. TOP scans your header information and RETR receives the rest of the message. This causes Avast email scan status window to show the same file being scanned twice. Check your option for email log file in your email client and you will get a better idea of what is going on when downloading messages.

If you have message rules set up in your email client than you will most likely have the TOP command when downloading messages.