Short answer: both Alpha GPC and Citicoline donate choline, a precursor to acetylcholine — the neurotransmitter most tied to learning and memory formation — but each has a somewhat distinct additional profile.
Alpha GPC: bioavailable choline
A highly bioavailable choline source, Alpha GPC is one of the more researched nootropics for supporting mental sharpness and physical performance specifically.
Citicoline: choline plus membrane support
Citicoline provides choline for acetylcholine synthesis and additionally supports phospholipid membrane repair — used clinically in some countries for cognitive support, a somewhat broader profile than Alpha GPC alone.
Why pairing two choline sources isn't necessarily redundant
Since acetylcholine production depends on adequate choline availability, ensuring sufficient supply through two complementary sources is a reasonable formulation choice — though it also means a higher combined choline dose to be aware of.
See both in Harmo Brain's formula
300mg Alpha GPC, 250mg Citicoline, disclosed plainly.
See the Ingredients