My company echoes everything CArthur7 (see above) just said. Allowing users to select/specify our desired ‘Unique Identifier’ property when importing contacts is exactly what’s needed. The majority of businesses have their own and those who don’t could default to Hubspot’s identifier.
The current way of deduplicating by combining/overwriting contact records makes businesses look foolish and inaccurate at best and opens certain businesses (like financial institutions) open to risk and liability if they send the wrong content to the wrong person, such as a minor.
It is a huge problem and is making my company research other options. A shame, because we love everything else Hubspot has to offer.