"You have already summoned this fae" message when switching Faen

In Brave Trials for Android, I have 2 Grade 5 Faens equipped as my Buddies and 2 Grade 4 Faens equipped as my Assists. I drew a Grade 5 Faen and wanted to put it in to my first Assist slot, switching out the Faen.

When I click on the Faen to remove from the Inventory screen, choose Switch Faen, and choose my new Grade 5 Faen, it states 'You have already summoned this fae', and does not switch it out. I can assign a Grade 3 Faen to replace my Grade 4 Faen, but this is not what I want.

How can I equip this Faen as my Assist? Using the Auto Equip button does not put it in there either, leaving the lower-grade Faen in.

Apparently, you cannot equip two of the same type of Faen at the same time. The Grade 5 Faen I'm trying to equip is also one of my Buddy Faen. Confirmed by purchasing a new Grade 5 Faen, and equipping that one instead.