set display-flagged-messages

SET DISPLAY-FLAGGED-MESSAGES YES/no  -  whether to show flagged messages

With the usual YES setting, flagged messages will be displayed (along with
  new messages) when you start MM, or GET or EXAMINE a mail file.

If you change it to NO, only new messages are displayed.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show   flag   headers
(at the MM> prompt:)  get   examine