you probably copied n pasted 1 or more units
1) delete all units in that group and start again
or
2) Look for a slightly darker colour unit icon in the mission editor, this would indicate 1 unit placed directly on top of another unit, following the use of copy n paste (simply "delete" that unit once, this will remove the top unit)
3) You could check your mission.sqm, you will probably find 2 units at the exact same location