Before slotted actions, it was only useful to stash an action once on a
data-block: that was all that was needed to keep the action's channels
associated with the data-block, and stashing it multiple times was redundant.
Therefore Blender would simply refuse to stash an action if it was already
stashed.
However, now with slotted actions the user may legitimately want to stash an
action more than one time: if the action has multiple slots, the user may want
to keep the channels of multiple slots all associated with a data-block. Doing
that requires stashing each action+slot combination separately.
This PR addresses this use case by now allowing each action+slot *combo* to be
stashed once. Stashing a given action+slot combo more than once per data-block
is still prevented, however, since that is still redundant.
Pull Request: https://projects.blender.org/blender/blender/pulls/133307
3ed2c2346d