We discovered that it was possible using the API to include hasUniqueValue=true in a Contact property's definition. Currently HubSpot doesn't support unique properties in Contacts. So despite the value showing as true, it was ignored and not enforced.
To rectify the incorrect data we'll be updating all contact property definitions that currently have hasUniqueValue set to true, to false. This correction will go into effect on Wednesday, April 12th.
This change will not affect behavior in HubSpot.
If you built an app or integration and coded it specifically expecting one of these properties to be unique, you'll want to update your code to expect that multiple records could have the same value for that property. For the vast majority of apps and integrations this likely isn't an issue.
If you use the API to create a Contact property and its definition includes hasUniqueValue set to true, you will receive a validation error. This specifically applies to property definitions on Contacts objects.