Numbers wise the maximum you could increase your loot would be to 2.4x (Normal loot + double loot + maximum prey bonus). In practice this means you could roll up three of the same rare item. It'll be interesting to see if someone if someone can in fact post a screenshot of the same rare item being received three times
Edit: Asked on my world and someone told me they had got 3 Terra Boots from a Ghastly Dragon. Not a rare as such but seems to indicate its possible.