I’ve seen lots of questions regarding NAV permissions but here’s another one that’s stumping me. If 2 permission sets have the same object but different permissions (i.e. read in one but insert/modify in the other and both are given to a user,
Hi,
Welcome to DUG! 
The highest permission wins! As long as they have INSERT permissions in a table in one permission set, then they have the access.
So each permission set adds more permission to the user. There is no way to say no-access or lesser access.
Could you please clarify what you mean by “highest permission”? Thanks. Mark
Yes, if you consider insert/modify as higher than just read.
But as I also write, permissions are just added, never removed.