Studios grow a second regular, and that is when series quietly merge: a hairstyle migrates, a signature detail turns up on the wrong woman, and two catalog cards become one average. The cabinet prevents it with cross-references — each card notes, in one line, how its character differs from the other.
The difference line is written when the second card is filed, and it is written in contrasts, not adjectives: where one is dark-haired and bobbed, the other is honey-blonde and waved; where one signs with a brass pendant, the other signs with a beauty mark. Contrasts survive generation because the model can hold two opposites more easily than two near-misses.
Practically: never prompt both cards in one job, never borrow a changing line from a sitting that belonged to the other character, and judge each frame against its own card only. When a blend does slip through, it is culled — not edited into a third woman nobody filed.
Two drawers, two cards, one difference line on each. That is all cross-referencing takes, and it keeps a growing cast distinct.