return order added two times one time as credit and debit proplem why

Hi guys i have problem when i make sales return order it come two time why 

example

order no            debit        credit

az-so-001       500

after i make return order it come as following

az-so-001       500

ret002            500

ret002                               500

total must be 0 after return but it show 500 as debit

it added one time as credit and one time as debit why this

the true way is to add as credit only

How i solve this problem and why this happen