Sharing uid/gid between multiple Linux users

It's very possible to create multiple users and groups those share the same uid/gid.
Seriously this may confuse everyone but sometimes you will really need to do that.
I won't tell my "why"; but I share how the thing would be done.

1. For groups, you need to modify 3 files /etc/group, /etc/group- and /etc/gshadow. What to modify? Well, you create a new line which contains exactly some same contents as the line that includes the source group, and update the line with your new group name. For example, let's say we have an original line group:bill.gates:x:1000:bill.gates that indicates the group bill.gates includes an user named bill.gates. Now create new line group:bill.fences:x:1000:bill.gates, and after saving the file /etc/group, you will have two groups bill.fences and bill.gates, both share the same gid=1000, and both includes the same user bill.gates :)
2. For users, you follow the same way, but changes land on 3 files /etc/passwd, /etc/passwd- and /etc/shadow.

Please mind the order of new entries. The very first line wins and some application which works with uid may only print the first username what matches the uid.

Source: https://github.com/icy/blog/blob/main/Linux.md