Block creation, when the record already exists

Hello everyone
I am in a serious problem, I have a custom module named ‘CONTRACT’, it have six major fields:
-NUMBER OF CONTRACT
-DEPARTMENT
-SUPPLIER
-START DATE
-END DATE
-CLIENT
When creating a contract, I would like that is there are fields verify, that way I DO NOT create a similar contract.

Thank you for your help and thank you for keeping me informed if you need other additional information.
Sincerely.