[5e] Can you learn spells that you already have as a racial ability?

Specifically, someone in my party is a Levistus Tiefling Warlock so they are able to cast Armor of Agathys once per long rest for free as a racial ability. It is my understanding that this spell hasn't been "learned" and therefore cannot be cast using a regular spell slot. The player would like to learn the spell as a Warlock in order to cast it using a spell slot in addition to the one free casting, but a couple of players in our group are saying that they cannot choose to learn Armor of Agathys as a regular Warlock since they already "know" the spell through their racial ability.

I've tried searching Sage Advice and other resources, but haven't found any actual runnings that govern this so if anyone could point us to an answer I would really appreciate it!