See the "Switch Port-Mirroring" section of chapter 6 of the Summit 200 Series Switch Installation and User Guide.
Extremeware 6.2.2 on the Extreme 3804 duplicates packets when running a port mirror. This may have been fixed in later versions.
Switch# config mirroring add ports 1,2,5,8 Adds ports 1, 2, 5, and 8 to the mirrored port list
Switch# enable mirroring to 9 tagged Configures mirror traffic to be sent to port 9, with VLAN tags intact
Switch# show mirroring Displays current port mirroring information
This is for ExtremeWare 7.0 and later.