I two command blocks set up as in the screen shots.
1. A repeating command block, with the command /say hi. This points into the second command block.
2. A chain command block, with the command /blockdata ~ ~ ~-1

. This coordinates point toward the repeating command block.

I expect the word hi to printed out once when I manually set the first repeating command block to Always Active.

However, it prints out hi twice, as shown in the last screenshot (21.33.12)

To recreate it, simply follow the instructions/screenshots.