One role commonly appears on the employer's careers page and several boards, sometimes with the text rewritten. Counting them separately inflates any measure of demand.
Exact matching catches only unmodified copies. Practical systems use a composite key: source identity where available, then normalised apply URL, then employer plus normalised title plus location, with description similarity as confirmation.
The hard case is a genuine multi-seat requisition, where near-identical listings are not duplicates. Every provider picks a rule; the useful thing is knowing which.