u/s.11 of civil procedure Code, 1908, Res -Judicata Means thing once decided cannot be re-agitated, more than one action dose not lie on the same cause of action. The doctrine of Res-Judicata rests upon the principle that one should not be vexed twice for the same cause and there should be finality of litigation.
The object of the principle is to prevent endless litigation. It also prevents a new investigation so that the same person cannot be harras again and again in various proceedings upon the same cause of action.
It is based on the maxim 'nemo debet bis vexari pro in et edem causa means no man should be vexed twice over the same cause of action.
See Also.... Difference Between Estoppel and Res Judicata