Hi,
No two person can work on same function module together.It will say its being currently edited by so and so user.
You can check the same.
Let say for EXIT_SAPLRSAP_001--transaction data
Double click on Include ZXRSAU01 and edit.
Try the same thing again it wont work.
Moreover Cmod has become obsolete and sap has stopped delivering new exits as well.
Regards,
AL