onkeyup fires twice for arrow keys only

Steps to reproduce

1. Assign focus to the text input below.
2. Press a single alphanumeric character on the keyboard. The event.keyCode of that key will appear once in the red div.
3. Press a single arrow key on the keyboard. The event.keyCode of that key will appear twice in the red div.

Expected behavior onkeyup fires once each time a key is released.

Actual behavior onkeyup fires twice when an arrow key is released.

This bug reported on WebmasterWorld by user mattx17 in this thread. Reported by whoisgregg.