No functional changes.
`TFM_TIME_DUPLICATE` is basically the `TIME_TRANSLATE` or `TRANSLATE`
modes.
The difference is that `TFM_TIME_DUPLICATE` merge duplicates keyframes
at the end of the operation.
It is an unexposed mode in python and promotes code duplication.
It has a similar initialization to `TIME_EXTEND` which redirects to
`TIME_TRANSLATE` or `TRANSLATE` depending on the spacetype.
As it is dependent on other modes (as well as `TIME_EXTEND`), it makes
things a bit more confusing.
There's no real benefit to creating a new internal mode just to
indicate this small change (merge duplicates).
So indicate in another way (by properties) that you want to merge
duplicate keyframes.
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