Under specific circumstances, cloning a command block while running it causes it to run twice.
How to reproduce:
1. Face towards positive Z (Bug occurs in multiple orientations, but the command to cause it is orientation specific)
2. Place a command block, and enter the command "/say bug" (or another command to be run twice)
3. Place another command block adjacent to it and between it and you (1 block towards negative Z), and enter the command "/clone ~ ~ ~1 ~1 ~ ~2 ~3 ~ ~1"
4. Again moving 1 block towards negative Z, place a redstone repeater facing the /clone command block
5. Again moving 1 block towards negative Z, place a redstone torch or other power source. "[@] bug" will be displayed twice, instead of the expected once.
Observations:
Clone source region has to include 1 block to the left (+X from this perspective) and 1 block behind (+Z from this perspective) the target command block. Including 1 block to the right or 1 block in front of the target does not cause the bug on its own. Clone target region makes no difference.
Replacing the repeater with redstone dust prevents this bug (displayed only once)
Bug still occurs if the two command blocks are separated by redstone dust, as long as the clone source region is adjusted accordingly.