There is not many solutions, beside removing completely that element (which can be dangerous in general, except in your case, this is about an obsolete branch).
cleartool rmelem
But a "safer" route would be to try an merge that obsolete branch, ignoring all changes except the addition of that file.
That way, you don't have to 'mkelem
' the same file again, you can reuse the one existing (and changing its content completely).