The principle of res judicata can be defeated when the party has filed the suit on a reasonable ground for example in case a public interest litigation has been filed there is no reason not to extend the doctrine of res judicata. The PIL has been filed with a bona fide intention and the litigation cannot end.

What are the Rulings on Res Judicata as Per Law
Res-Judicata not only ousts the jurisdiction of the court but a person cannot be harassed again and again upon the same question. A judgement delivered by a court of competent jurisdiction operates as a bar as regards all findings which are essential to sustain the judgement.