Question:

Why did Mormon give Moroni his abridgment twice?

Answer:

It appears that Mormon surrendered his almost completed abridgment to Moroni in 385ad, immediately prior to the battle of Cumorah.  Mormon was left for dead on the battle field, but having survived the battle, his son returned the plates to him.     Mormon kept the plates until around 399ad, where in the 9th chapter of Moroni, he requests that his son return and get the abridgment for the last time.