double check

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A careful reinspection or reexamination to assure accuracy or proper condition; verification.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An extra check or verification made a second time, usually with additional caution or attention.
• noun chess, uncountable A situation in which a king is attacked by two of the opponent's men at once.
• verb To check again; to check with extra caution or attention; to verify or make sure.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun something that checks the correctness of a previous check