This can happen if you copy or move a package.
In case you moved a package, it can be a false positive, so make sure to rescan for packages.
Otherwise, it's simply as the error states. Two packages contain a "composer.json" file with "name": "neos/demo". Search for it to find it and fix it.
Btw. in case you copied the neos/demo package as a basis for a new package, make sure to remove the original neos/demo package using this command to avoid conflicts.
composer remove neos/demo