Board Thread:Questions and Answers/@comment-1814996-20130622124601/@comment-1814996-20130622140747

I found the answer. It's additive, which makes sense, but you never know. After all, Tom snags deeds and that doesn't make much sense at all.

Here's the answer from the forum FAQ.

When you own more than one of the same kind of production buildings, total product times are combined! For example if you have two Smelters and produce 1 gold bar in each smelter, you will total 24h of production time (12h/gold bar). If you receive a star for one Smelter, you will get a star for the second Smelter as well! You will still earn production hours towards mastery when you spend diamonds to speed up production times!