Definition: A duplicate bug is a bug report describing a problem that has already been reported. Duplicates usually appear when someone finds an issue, cannot quickly tell whether it is already in the bug tracker, and raises it again. Most bug trackers have a duplicate status so the newer report can be closed and linked to the original.
So what? Every duplicate brings its own investigation, discussion and re-testing for what is really one problem. The more unfixed bugs sitting in the tracker, the slower it is to check for an existing report, so people check less often and duplicates become more likely.
Examples: With 5 open bugs, a tester can scan the list in seconds before raising a new one. With 50, checking takes long enough that it gets skipped and the same issue is reported twice. Two testers working on different parts of a checkout flow each report the same broken payment button, and both reports are investigated before anyone spots the match.
