[snarky]

From the thread linked by MM above:

Originally Posted by MunchkinMan

Mate creates a separate Monster which actively duplicates the original. Illusion, the last card you referred to, can be played on either the original or the Mate (since they are both separate Monsters, regardless of the link). If you replace the original Monster, the Mate duplicates the new Monster.

So, if you use Illusion on the Mate, you turn the L20 Net Troll into a L12 Wannabe Vampire; the original remains in play as an L20 Net Troll.

If you use Illusion on the original L20 Net Troll, you are facing two L12 Wannabe Vampires.