Independence is not a property of a source's brand. It is a property of the pipe the source is drawing from, and the pipes in this industry are shared far more than the vendors on top of them admit. Two feeds that resolve, on inspection, to the same upstream data licence are one source counted twice, and weighting them as independent is a quiet, expensive way to be wrong with confidence.
We map lineage on every source; where does this feed originate. Who sells it downstream; which vendors we already use are secretly this feed under a different label. When two of the sources in a corroboration turn out to share a parent, they collapse into one for scoring purposes.
The lineage map is the least glamorous artifact we maintain and the one that changes our numbers most. It is also the one no vendor will help us build, because the incentive runs the other way. We maintain it internally, quietly, and update it every time a new source comes into the stack.
The lesson generalises past monitoring; any system that combines opinions has to know which opinions were formed independently. Skipping the check is fast; paying for the skip is slow and expensive later.
Two sources agreeing feels like confirmation. It only is if they arrived at the same answer independently. If both drew from the same feed, the agreement adds no information; it is the same source counted twice. Systems that don't track upstream lineage overstate their own certainty by exactly this margin.
The habit worth borrowing, outside any analytical setting, is asking of any consensus how many genuinely separate paths of reasoning it rests on. Most consensuses look sturdier than they are because the same input reaches many outlets and each one is read as a separate voice.