I thought you could mirror the selected mesh by shift + D to duplicate then Ctrl + M to mirror then select which axes to mirror the duplicate.
Ctrl-M is for mirroring in Object Mode (M is reserved for moving an object to another layer) . Use just M if you are in Edit Mode to mirror a duplicated mesh .
OK, now when I select the mesh in edit mode A to select all, shift + D to duplicate, M to mirror and select Y axes shouldn’t it flip the selected mesh on the Y axes ?
Yes . I just tried it and it works . Is it not working for you ?
EDIT : You need to hit enter after you Shift-D . The default behavior is to translate/move the duplicate . Hit enter then hit M then Y .
OK . I thought I could dup. said mesh then while its selected then M to mirror on y.