DHL often pay customs duties on packages so that they can be delivered promptly, and charge the fee's at a later date, however, when they do this they'll send you a DHL letter asking for the charges, not one from a debt collection agency.
i suppose the pertinant question is, have you recieved anything that has been brought in internationally by DHL in the last 2 years?
anyway, if you have, just give 'em a call and ask.