Duplicate TAN is a TAN which has been inadvertently obtained by a person who is responsible for deducting/collecting tax and who already has a TAN allotted to him. It is illegal to possess or use more than one TAN.
In case duplicate TANs have been allotted, the TAN which has been used regularly should be continued to be used. The other TAN/s should be surrendered for cancellation using 'Form for Changes or Correction in TAN' which can be downloaded from the NSDL-TIN website.
However, different branches/divisions of an entity may have separate TANs.