Some cities producing 2 for 1 units

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.

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!