Ubuntu used a patch for this, but it's just a work-around for now and multiple upstream's developers told me that is not the right approach to use. -- debbug#555134 ...
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ID Status▲ Resolution Sev▲ Product Comp Assignee▲ Reporter Summary
17720 NEW --- enh Sisyphus shadow-u sem imz if the user already exists, useradd could check for compatibility with the request

File a new bug in the "shadow-utils" component of the "Sisyphus" product