D&D 5E How to deal with a Warlock gaining Detect Magic as a cantrip

[Fanaelialae]

Actually, this sounds like it might be an interesting plot-hook for a Warlock PC... Twin Warlocks separated at birth... The Warlock PC's ability is the essential thing that enables the PC to follow the breadcrumbs of permanent Magic Auras to find their Warlock NPC twin.

Then only one question will remain... Which one is the Evil twin?

I'd just make it a bonus cantrip unique to the twins, kind of like how some twins invent their own languages.