Use Exclude Left End Point to exclude the beginning key in a selected block of time.
If you paste the same block of time repeatedly, one block following the other, you can create a looping segment in your animation. To create a smooth looping animation, you need to exclude either the first or last key of the copied block to prevent keys from doubling up at the ends.
The key to be excluded must be at the exact start time of the copied block of time.
To animate a loop by copying and pasting keys:
(Select Time ) to specify a block of time.
At least one track in the block should start with a keyframe. Also, for a smooth loop, the first and last frame of the selection should be the same.
Extras: Dope Sheet.
The Exclude Left End Point and Exclude Right End Point buttons appear.
(Exclude Left End Point).
(Copy Time).
The selection is copied to the clipboard, minus the first frame.
(Paste Time).
Play your animation to observe the effect.