what is the procedure and what supporting documents do i have to submit for a Double entry visa to China?

What purpose of your visit to China? if for business, you need an invitation letter from China, in which your stay time, your personal information, inviter's ID,and company's business registration should be written. If for travel, you need to provide return ticket and hotel bookings, itinerary confirmed by a local travel agency or bank statements for travel. Just fill your double entry visa requirement to the application form, the consular officers will take it into consideration.