Hi,
You cannot repeat same name for source field. Give a different name to the credit side posting key and other relevant master data. As name of tje step suggest it is source field, if you repeat the name than you cannot pass two different value in upload file. I will advice you to go through below wiki link Ravi has nicely explained the same:-
LSMW for Functional Consultants in simple step-by-step way
Regards,
Anand Raichura