two Pit Strider minion Bug Possibly

I have been using Infernalist minion build and I collected two Specter Pit Strider minions. They have a lot of health but there offense severely diminished but they are fun to run around with.

I have 237 spirit they both require 99 each to used. I had no problems until i removed one by mistake. When I attempted to re-slot the gem I am not able to.

There is no information in the tool tips of the gems to suggest that there is a limit.

Is this a bug?