The AI tends to build multiply mining starbases for the same resource. It appears that if there are constructors on there way to a resource where the AI wants to build a mining base but before the constructor gets there. And either another player or one of its own constructors has already built a mining base in the mean time while its traveling. it still looks for a location to build a mining base and does not check to see if the resource has been claimed.
 
Pic 1:
 I noticed it in my game. I built a fast constructor to beat an AI getting a resource I wanted. I bet it there and took it, then when the AI constructor caught up about 4 turns later it tried to find a spot then just built a mining base on the resource even know I already claimed it.

Pic 2:
 In this game I started and soaked it (I have the save if you want it), I have tried it three times now with this save and each time the AI did the exact same thing. Each time it sends constructors to the first location, a few turns later the second constructor arrives and it builds it. then a few turns later the 3rd arrives and it builds it in a hex where the resources are taken already, in fact it completely missed the resource it COULD have gotten just one hex higher. You cant see but they are ALL mining starbases.