https://etias.com/etias-frequently-asked-questions:
I have dual citizenship for an E.U. country (e.g. Italy, France, Germany, etc.) and for an ETIAS eligible country (e.g. USA, Canada, Australia), do I need an ETIAS? No, you should use the passport of the E.U. or ETIAS member country to enter and exit Europe. You will not need an ETIAS if you travel using the passport of the E.U. member country.
Is that a legal requirement? What are the legal ramifications if a dual citizenship for an E.U. country (e.g. Italy, France, Germany, etc.) enters an E.U. member country not using their E.U. passport?
My main interest is French citizens.