Uniqueness of a form

Dear Users

In a form, how to make a field or group of fields unique.

For example, in a mobile, I have two question ids namely site and pid.
Sire has length 1 and validation of range from 1 to 5.
Pid can be a number starting from 1 to 1000.

I want, if from site= 1 and pid =0001 has been entered/submitted to server,

User can not enter the same site=1 and pid=0001 again.

I mean the uniqueness of a form by site and pid.

How this can be done in commcare?

Thanks and Regards