Steps to Reproduce:

    1. Place two unpowered repeating command blocks a block apart.

    2. Insert the following command into the first command block:

/say 1

       And in the second command block:

/say 2

    3. Place a redstone block in between the two command blocks, powering both at once.

Expected Result:

    In chat, 1 and 2 should alternate consistently every tick with no repeats.

Observed Result:

    1 or 2 sometimes appears twice, indicating the other command didn't activate that tick.