Combination field Unique

Hi,

As per our business requirements, we have 2 fields (Ex: AccountId, ContactId) stored the other object Id values. We are inserting the records from external systems.

Is there any standard configuration available to avoid duplication when we try to insert the record in same combination (Ex: AccountId, ContactId). We expect the Post method should errored out when we try to insert the record with same combination. In siebel we have User Key option at BC level.

If we dont have any standard option, then we have to use Get option first and based on the response we need to use Post or Patch method.