How will the person prove that the principle of unjust enrichment do not apply in his case?

The person concerned may furnish together with the
application such document(s) or evidence(s) to establish
that the amount of tax and interest, if any, paid on such tax
or any other amount paid in relation to which such refund
is claimed was not passed on by him to any other person –
section 38(3)(b).
Further, to provide relief to taxpayers the above sub-section
also provides that where the refund amount, as claimed, is
less than Rs 5 lakh a self-declaration will only be required.