Posted on 05 July 17 at 07:30
Unlike the xbox 360, Playstation 4 trophies do not automatically sync with the server
You need to actually view the trophies for them to sync.
If you didn't do this on your friends console then that's why you were able to unlock the achievement again.
Basically you should get in a habit of viewing your trophies after every gaming session to get them to sync.