You're absolutely right. That's why I'm simply proposing a ban on voters signing multiple nomination papers, along with a requirement that a nomination can't be affected by the inclusion of the same signature on more than one nomination paper. If a voter signs more than one nomination paper as a result of absent‑mindedness, malice or any reason whatsoever, the returning officer won't reject the nomination papers. In any case, we can't know the order in which the papers were signed.
I'm not proposing a measure that challenges the validity of the nomination paper. However, people must be prohibited from signing more than one nomination paper or from encouraging others to do so, and penalties must be imposed on offenders.
The validity of the signature isn't being challenged. People are being told that, if they do these types of things, they'll face penalties. The candidates aren't affected.