The middle-aged imitators both operate in the eastern town of Epinal in the Lorraine region where they have maintained a fierce rivalry over several years.
Until now, that enmity has been restricted to the odd verbal provocation and occasional jokes as the two tour the circuit of local events and competitions.
Local newspaper Vosges Matin reports that events took a more serious turn on Saturday when 'Gainsbourg', otherwise known as Denis C, 46, saw 'Hallyday', Michel P, 50, mowing the lawn of the apartment block where he lives.
Leaning from his upstairs window, Denis took the opportunity to taunt Michel who apparently gave as good as he got. Whatever remark he made clearly upset Denis to the point that he allegedly grabbed a knife, approached his rival and plunged it into his neck.
The Johnny Hallyday impersonator ended up in hospital. His attacker was arrested and now faces charges of attempted murder up to 30 years in prison.