Which Unique ID you are talking about?

If you are asking for Adhar card,then please follow the following procedure:

If you have lost your unique identification Number (UID)/or Adhaar Enrollment ID(EID) or even their slip,You can get it by 2 ways-one by visiting it's website or by dialling 1947.To retrieve it online,one needs to visit the official website of UIDAI:uidai.gov.in. Once you are on the website,look for 'My Adhar section'.There you will find 'Adhaar Services'.Click on 'Adhar services' tab,You will be directed to retrieve lost or forgotten EID/UID.Go to resident.uidai.gov.in/lost-uideid section to simply fill the details in order to retrieve your lost/forgotten UID and EID.Your registered Mobile number/Email which you had provided at the time of enrollment is mandatory to retrieve your lost UID/EID.