If you accidentally clicked the "Create pull request" button multiple times, you may end up with creating duplicate pull requests (just like I did in #3274 and #3392 馃槄).
The button should be disabled right after you click it, just like "Comment" and "Update comment" button, but somehow GitHub doesn't enforce it (or should it be considered GitHub's bug?).
I don't know how often this could happen (or it's just me who suffers from this 馃槀), but I think it makes sense to disable the button after you click it, to prevent the problem.
EDIT: I remembered it wrong, "Submit new issue" button doesn't have this issue.
Please also report this to GitHub.
Most helpful comment
Please also report this to GitHub.