belt and suspenders
expressionops culturesecurity
Wearing two independent safeguards for the same failure - if one lets go, the trousers stay up.
The idiom for deliberate redundancy of controls: backup and replication, MFA and conditional access, dual power feeds. The word 'independent' carries the weight; two straps anchored to the same failing point are one strap with extra steps.
Belt and suspenders describes using two mechanisms where one should be sufficient, and the phrase carries a slight implication of excess that is worth examining rather than accepting. In safety and security work, the redundancy is usually the point rather than an indulgence.
The justification is independence. Two controls that fail for different reasons produce a combined failure probability far lower than either alone, which is why a firewall rule and host-based restriction, or input validation and parameterized queries, are both worth having. The second one exists precisely for the day the first is misconfigured, bypassed, or found to have a flaw.
The version that deserves the sceptical implication is redundancy that is not independent. Two checks in the same code path, both written by the same person on the same day from the same assumption, fail together and provide false confidence while costing complexity. That is the distinction to apply: ask what would have to be true for both mechanisms to fail simultaneously, and if the answer is a single mistake, you have one control and the appearance of two.
Also known as: belt and braces