Detect double `Refs` and `Fixes` entries.

[BridgeAR]

Right now if the tool is used as it the commit message that is generated duplicates Refs and Fixes entries by amending those blindly. We might come up with a way to remove those - and verify that they are identical with the ones being amended (and if not, it should result in an error).

[joyeecheung]

BTW https://github.com/nodejs/node-core-utils/pull/106/files#diff-806602ca40b9756be13878981c68b536R109 does handle this in case there are already urls present in the commit message without metadata.

[joyeecheung]

It's fixed by the new git-node command, although it still does not fix the following case:

Fixes: #XXXX   # put by the author

Something something...

Fixes: https://github.com/nodejs/node/issue/XXXX # generated by us

[BridgeAR]

@joyeecheung that was the case that I meant ^^.

[github-actions]

This issue is stale because it has been open many days with no activity. It will be closed soon unless the stale label is removed or a comment is made.

[codebytere]

Will be closed by #478