How to you master an item more than once

When you defeat an enemy, you are awarded a fixed amount of experience for each unmastered item you had equipped in the fight, and for each item, that much XP goes toward moving your character to the next level. The total XP is essentially a multiple of your unmastered items. So if an enemy is "worth" 500 XP, you'd get 2,000 total XP if you are using an unmastered weapon, shield, armor, and helm.

If an item is Mastered, you no longer gain experience for using it. So, if one of the items you use in a fight is mastered, then the portion of the XP earned for that item doesn't go toward moving your character to the next level - it is effectively lost.

IMO, the only time you'd want to use a mastered item over an unmastered item is if you are having a hard time with a fight, and the mastered item is stronger than the unmastered item.

You can start a New Game+ by beating the end boss in the dungeon (accessible by moving to the open hatch, just before the door to the castle). Once inside, if you have the Infinity Blade you'll unlock more of the dungeon, eventually getting to the end boss. Once you defeat him, the game gives you the option of starting a new bloodline or doing New Game+.

Update: I originally posted that you could start a New Game+ any time in the Options. However, I've had a chance to start the game up but I couldn't find anything there that indicates this. I also searched online, and was unable to find anything there as well.