This Bible contradiction is from the Skeptic's Annotated Bible.

Verses

No, it was never rebuilt after Joshua destroyed it.

Joshua 8:28 View context

And Joshua burnt Ai, and made it an heap for ever, [even] a desolation unto this day.

Yes, it existed at the time of the Babylonian captivity.

Nehemiah 7:32 View context

The men of Beth-el and Ai, an hundred twenty and three.