RESTRICT USER FROM USING SAME REQUIREMENT TYPE WHILE CREATING SALES ORDER

Hi Friends,

I Have a requirement to in SD while creating any sales order i want the system sholud throw a error

message when i am using line items having same requirement type more than one time

i have tried some customer exits v45a0001 & v45a0003 . but was not successfull .

kindly somebody help me with elaborated steps .

thanks in advance .

regards

digvijay rai