Are special resources of the same type cumulative?

I think how it works is that the total mining of all like resources is added together. For instance, if you have one starbase mining 15% economy, and another starbase mining 20% economy, you get a total 35% economy bonus.

Peace & Luv, Liz

Yes they are cumulative. Nearly every bonus in the game is cumulative. If you build nine economy bases next to a planet you can get an assload of bonuses. Of course nine starbases being updated in addition to your other starbases can get expensive. I typically build 2 starbases that can reach a couple of my most important planets so I can get double bonus on those important planets. This is of course assuming that my situation allows me to afford that. It is a large up front investment for a per turn bonus.