How to check unique validation according to another db column value?

from above table I have to validate regd_no according to to_branch_id. Let's say if I got to_branch_id = 1 then unique will be apply for regd_no but if I got to_branch_id=2 then I need have duplicate values from column to_branch_id = 1 as well as unique if another record is enter in to_branch_id=2.

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