1 - Go on a jam submission page, such as this one : https://itch.io/jam/jame-gam-20/rate/1640834?before=13#post-6360351

2 - Rate the game, but dont save your changes

3 - Write a comment

4 - Try to post it

5 - Firefox says "are you sure you want to leave the page ?"

6 - Choose "stay on page"

7 - Notice you didnt save your rating

8 - Save your rating

9 - Try to post your comment again

10 - Boom, you got two comments that are identical