WordNet (r) 3.0 (2006):

double check
    n 1: something that checks the correctness of a previous check
         [syn: countercheck, double check]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "double check":
   audit, balance, balance the books, check, check and doublecheck,
   check out, check over, collate, confirm, cross-check, demonstrate,
   inventory, overhaul, prove, recheck, take stock, test,
   triple-check, validate, verify