This should hopefully be an easy question:
Let's say User#1 has explicit Modify permissions to a specific shared folder.
We make a copy of User#1 and call it User#2.
Will User#2 also have explicit permissions on the same share?
No, permissions are based on the SID (unique identifier assigned to every users, groups, etc... )
The newly created user will have a new SID.
You should put your users into groups, and then give permissions on the group instead of users.