>Can anybody certify that the SYS(2015) function is generating unique ID. I plan to use this as a primary key in a big system and I want to be sure it won't generate duplicate...

I understand that it's only unique on a given CPU/clock. So it's possible that two users may generate the same number in a network application running on local PCs.
A solution may be to incorporate the users ID into a function...
eg

FUNCTION UniqueID
RETURN oU.userID + SYS(2015)
* where oU is some user system object