Prevent the insertion of duplicate records

Question

I want to prevent the insertion of duplicate records,if the record is already exists then I want to send a message the record already exists.

please find the below file,It contains my database image.

I want to insert the data through regid,diffrent users can enter the same data but when the same user trying to enter same record 2 time then I should prevent.
   In the below picture I rounded with blue(Regid:45),the user can able to enter like that but when the user is trying to enter same details with same regid(45) then I should restrict.Can any one tell me how to write the code for this