User blog comment:Miskos3/Fairy Tail 528: Dragon of Magic, Review/@comment-86.70.179.109-20170325114651/@comment-7418318-20170330211204

Zodiacs are unique. There is only one of each key and they can only be contracted to a single master at a time.

But we don't know if the contract holder can loan the key to another Celestial Spirit mage or if that would requite the consent of the Spirit.

Yes Spirit's strength depends on the strength of the summoner, but we don't know if Anna was any stronger than Lucy is right now. The only thing we heard is that she was "incredible"... but that can mean anything.

And Lucy's become pretty powerful too. Even back in Tartaros she was able to temporarily hold her own against two Demon Gates and a third Demon of lesser grade, despite already being hurt and exhausted from earlier fights. And that was before the 2nd timeskip.