What duplicates actually cost
The visible cost is embarrassment: two sellers contacting the same buyer, or a customer receiving the same campaign twice. The larger cost is invisible. When one buyer exists as three records, the activity history that would have told you they have been in touch three times over eighteen months is split into three thin stories, none of which justifies attention. Your counts are also wrong, which means conversion rates and source performance are wrong, and those numbers drive budget.
It is worth being clear that perfect deduplication is not the goal. A small residual of genuinely ambiguous records is normal and cheap. The goal is to remove the systematic inflow and the obvious duplicates, then keep the remainder small enough that it does not distort decisions.