I conclude that you cannot get Rachelle by defeating Kerrachelle

 ...after getting Kerra 5 times. 5 out of 5 is statistically significant enough for me.

Now, what do I do with all these Kerras? Make an army of Kerrachelles? It's not SuperRare but the stats are very good for a 4-star. Just need to hope that it scales well as the spellbook gains levels. Easy to evolve too (no 3x rare evo books like certain spellbooks).