Can a creature be equipped with multiple artifacts?
If so, can several of the same one, such as Mask of Avacyn, be used?
Yup, equip as many of them as you like.
Note that you need to be able to target your creature to equip it. So cards that give your creature shroud (like Whispersilk Cloak) can get in the way of stacking on lots of equipment.
The Mask is fine, since it gives Hexproof instead of Shroud.