Wiktionary
belt and suspenders
n. redundant systems, affording mutual backup in the event of one failing.
Usage examples of "belt and suspenders".
Another of his peculiarities was that he always wore both a belt and suspenders.
He wore khaki pants with both a belt and suspenders, and a blue work shirt with a black bow tie.
But considering what became of Ground Station Hesperus, Gomez and Tev are recommending we wear a belt and suspenders both.