define "same subnet"
Define what is "same subnet" i.e. what makes two similar subnets allowed or forbidden.
Obviously same subnet ID should be forbidden.
Same subnet text representation (i.e., 192.168.0.0/24) is more arguable.
Note this is a design/discussion ticket (no branch, no code, no doc).
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information