My iPhone 4 won't charge but the same charger is working with my iPad please help what could the problem be?

I am using a iPhone 4S and I'm using a duplicate charger not the original charger. I use the same charger to charge my phone and it's working perfectly. It has been working with my iPhone as well but it recently started giving me problems and giving me the message that the accessories used could not be supported. Please help? What can I do?