Repiping a 40 m kcp  came new with single wall would like to replace with double will the weight difference be an issue   Or should I go back with single 

I assume you have DN125 (4.8") single wall pipe on your boom, therefore either a 3.00+1.5-mm or 2.5+2.00-mm (wall thickness) twin wall pipe will work. Give us a call at 262-732-4109 

The weight is regulated by the manufacturers but that doesn't mean the people making the pipe follow the recommended weights. They should be the same for single wall and TW pipe. Some Asian pipe manufacturers do not adhere to the guidelines. For me the defining factor is the quality of the operator. If it's a newer guy that has boom party issues go with single wall so you can bang on it if you have to, to get it clean. If it is an experienced operator, then use TW pipe as you shouldn't beat on the pipe and break the linings. (This way you don't lose the additional money you put in the pipe.)