G/L - Item Ledger Relation already exists. G/L Entry No.=0, Value Entry No.=1027

Hi. I am referring to NAV 2016. My customer facing a problem whereby when click Preview Posting button in Sales Invoice, error prompt: G/L - Item Ledger Relation already exists. G/L Entry No.=0, Value Entry No.=1027." I have checked the G/L - Item Ledger Relation table & Value Entry, the Entry No. does not exist. However, when I click Post button, no error prompt, the Invoice posted successfully.

I have no idea how is this error prompt, and where else should I check. Kindly advise. Thank you.