Elon's Lift is claimed to be one per galaxy.  In one of my games it appeared on my list of things I could build several times.

I'm sure it was built earlier, but I can build it now on Turn 176.

It's not easy to see, but it's now Turn 178 and an AI has built it.

It's now Turn 194, and it again appears that I can build it.

I've checked the ImprovementDefs.xml file and found that it is marked as indestructible, so the AI shouldn't be able to destroy it.  I'm not sure if conquering a planet will cause this.  I expect if a race surrenders and its planets don't transfer, this could happen.

I'm reporting this so everyone who reads this will be aware that this happens.  Maybe someone will get more information or a save before it happens so it can be determined if this is normal or a bug.  I will be more aware the next time this happens to  me.