I HEAR so many different opinions for lpg.Does lpg destroy an engine or not?

A badly done LPG conversion can give you lack of power, and possibly engine damage, but that's the conversion rather than an intrinsic issue with LPG. (sources Citroen Car Club and Dave Walker)

The spark plugs designed for lpg give something better in car performance or not?

I've never heard of spark plugs specifically designed for LPG. It's possible that an LPG converted car that's basically never run on petrol other than for starting would benefit from a different heat grade of plug (that's the number like, say, LB7HD on the plug insulator) but different engines sometimes run better on one brand than on another even on petrol, so you'd need expert advice on the specific engine, tune and LPG to answer that properly.