Currently (Jan 2016, API 2.0) using the API to PUT a new contact fails when its email is in use by an existing contact - as would be expected.

However, if the contact that has the duplicate is archived, the PUT will SUCCEED, returning a new contact ID and everything. However, the new contact will not actually be added. This can be done repeatedly, each time "generating" a new contact ID.

Given this, there is no way to ensure that a given contact has been added via the API before attempting to, for example, create a new Invoice via the API. It will silently fail on contact creation, then loudly fail on invoice creation.