In an image like post #4661649 where it's the same character in different forms, but in the same composition(so not like model_sheet or multiple_poses), do I tag solo or duo? dual_persona+solo and dual_persona+duo both have a lot of hits...

square_crossover usually counts each instance of a character as a separate character, so duo, trio, or group if there's more than one version of a character.

if the instances of the character appear to exist in the same space simultaneously it's square_crossover and not solo, if there's some separation via panel lines or similar that implies that there's like a transformation from one to the other than it's solo.

I'd say that the example image falls into the square_crossover category.

without any context it is just 2 distinct bodies sharing the same space and as such should be tagged duo (+square_crossover by sipo and wat's reckoning)