Cancelling a will procedure

Hi,

I would please like to know...

If some one makes a will of ancestral land on X name and then after several year he makes another will of the same land on Y name then can X challenge the will made on Y name after the will maker is dead.

I was told that the second will is the final and last but does the first will holds no value at all?

What should be the most strong points which the first will holder should be looking into his will and what weak points he should be looking on the second will to challenge the second and final will.

Thanks,

Rajendra...