I think since the very early days in Jira Data Center, custom field names have never been unique.
Yet, all over the interfaces, you have selection for custom fields that show nothing but the name.
This is from CSV import (old experience): 
One Team is my custom field, with its context restricted to my space only. The other Team is a locked field so I can't get rid of it even if I wanted to.
You have the same problem in Space settings | Screen as well.