Flanking is a keyword ability.
Amy is attacking Noel with a Benalish Cavalry and Noel controls a Benalish Cavalry and a Canyon Minotaur. If Noel blocks with the Canyon Minotaur, flanking will trigger and shrink the Canyon Minotaur to a 2/2 until end of turn. However, if Noel were to block with the Benalish Cavalry, flanking wouldn't trigger, as the blocking Benalish Cavalry also has flanking.
- 702.24a. Flanking is a triggered ability that triggers during the declare blockers step. "Flanking" means "Whenever this creature becomes blocked by a creature without flanking, the blocking creature gets -1/-1 until end of turn."
- 702.24b. If a creature has multiple instances of flanking, each triggers separately.