Double Counting

Double counting is recording the same thing more than once, and it inflates numbers in ways that survive review because each individual record looks legitimate. The sources are mundane and they compound. A delivery is retried after a timeout that had actually succeeded, so the event lands twice. The same action is instrumented on the client and again on the server, and both streams are loaded into the warehouse. One person signs in from a laptop and a phone and is counted as two users until their identities are stitched. A user belongs to three workspaces and appears once per workspace in a report that claims to count people. Revenue is recognised when a deal is booked and again when it is invoiced. A join against a table with more than one matching row multiplies every fact it touches, which is the quiet version and the one that produces the largest errors. The strongest tell is arithmetic that should hold and does not: segments that sum to more than the total, a count of distinct users that exceeds the number of accounts, or a figure that jumps on the day a new integration was switched on rather than on the day anything about the business changed. Prevention is mostly about identity. Give every event a stable identifier generated where the event happens, so a repeat delivery can be discarded on arrival rather than reconciled later. Decide for each metric which system is authoritative instead of loading two sources that both look plausible. Define the unit being counted in words before writing the query, since person, seat, account and workspace are four different denominators. And make the sum-of-parts check part of the report rather than something someone thinks to run after a number looks surprising. Deduplication after the fact is always harder than it looks, because by the time anyone notices, the inflated figure has been quoted, planned against and compared to a prior period that was counted the old way, and correcting it produces a fall that has to be explained as an accounting change rather than a business one.