I didn't see any posts about it, so I decided to compile some screenshots with two different occasions with two different people, using different weapons as well.
All I did was attack someone Duke wanted me to attack. It popped up in every attempt I made to defeat them. After I did, I decided to see if it would happen again with someone else. It did.
I use the latest Google Chrome, Windows 10, Pc.
Screenshots:
First occurrence - https://gyazo.com/30f679d9f52e74c76470c66a72012816
Second - https://gyazo.com/e73f2317c4cca3faa3cba3ebce46a084
Third - https://gyazo.com/0216df86cc28c597c9a3cc3d9c05c9c4
Fourth - https://gyazo.com/1d23cf7738a5e77224718b594eac6018
I haven't had the energy or time to test it out on anyone else, just thought i'd quickly post what I saw.