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. Itll 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.