i have posted a credit memo but the document is showing in both un posted and posted.
if you did not change the standard code then you did not close the CreditMemo fully. Check Quantities for each line.
P.S. I received the same issue sometimes when NAV received the error during the posting process. but it was in NAV 5 and current version should work OK.
Is it possible you posted that credit memo partially? If you check the quantities invoiced, do they match the quantity field for all the lines?
You need to post the Return Order partially, but not CreditMemo