This is in continuation of what Mr.KK!HR has rightly observed.
The questioner's statement that no more possibility of any further settlement on wages since 5 settlements were already concluded combinedly read with the no of trade unions and the no of settlements which are also incidentally five gives me an impression whether the management signed separate 5 settlements on wages having the same terms and conditions separately with each union. At times it so happens because of inter union rivalry and the management's anxiety to implement the settlements effectively.
On the contrary, had all the settlements been bilaterally signed for different periods of successive duration with any one of the unions based on the then prevailing majority of membership u/s 18(1) of the IDA,1947, the possibility of fresh charter of demand by any other union due to realignment of membership of workers during the currency of the last settlement cannot be ruled out. Of course the validity of such a fresh dispute on the same issues covered by a subsisting bilateral settlement can be decided in adjudication only based on the fairness of its terms and subsequent material changes in the circumstances. Therefore, strictly there cannot be any statutory ban on such a move of any trade union.
Besides, as an employer or a HR manager, the questioner should recognize the fact that wages cannot remain stagnant; the real value of money wages vary according to the fluctuations in the general price level. Workers will always demand more in terms of wages and other employment benefits which can be quantified in terms of money. It is for the employer to maintain a balance between the hike in the cost of production and the corresponding raise in productivity through innovative strategies.