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.
Use the exclude endpoint buttons to exclude the beginning or end key in a selected block of time.
Exclude Right End Point: Prevents a key at the last frame of a selected block of time, from being copied to the clipboard.
For a smooth loop, the first and last frame of the selection should be the same.
The selection is copied to the clipboard (minus the first frame).
The last position key should be your insert point.