My cities have produced 2 for 1 units. Aircraft carriers, destroyers and modern armour. A few times.
I have looked around and cannot work out how this can be happening.
Perhaps it is the civics? +100% production. but then again, there is 50% production.
Maybe this is a bug.
Playing emperor difficulty. Have harbour and encampment districts and many cities.
Best Answer
I found the Venetian arsenal in one of my not-so-unit-producing cities.
Since the civopedia is incomplete and vague I thought I would add this answer.
The Venetian Arsenal empowers any city building a naval unit to produce 2.
Untested, but if true; A powerful wonder if building armada level units!