In the animation transform code, `T_AUTOMERGE` was used
to indicate that the duplicated keys need to be merged.
In perparation of #111051, this no longer makes much sense because
in grease pencil we cannot have two keys in the same channel at the same
frame.
This PR replaces the `T_AUTOMERGE` flag in the animation code to
`T_DUPLICATED_KEYS` to indicate that the transform is being called
with duplicated keyframes.
