github: commits incorrectly signed off

Question

Anytime I open a PR on github I have to resolve this failed check:

 "commits incorrectly signed off"

I don't understand why. Is there a command to verify the signoff before opening a PR?

Looking at the github output:

Commit sha: 5daed5f, Author: gianpaolo, Committer: gianpaolo; The sign-off is missing.
Commit sha: c76c1a1, Author: gianpaolof, Committer: gianpaolof; The sign-off is missing.
Commit sha: 5e7b46f, Author: gianpaolo, Committer: GitHub; The sign-off is missing.

Look like there are 3 different committer? Thanks

Answer 1

Since my commit history is short and i have not shared work, this is what I did:

1) remove all signed off

    git filter-branch --msg-filter "sed /^Signed-off-by:/d"

2) git push -f