Why ZX_LINES table includes duplicate lines for PO transactions

Summary:

Why ZX_LINES table includes duplicate lines for PO transactions

Content (please ensure you mask any confidential information):

Our ZX_LINES table includes two lines for the same PO.
The two lines includes an identical TRX_ID and TRX_LINE_ID.
But these two lines includes a different values in the TAX_LINE_ID. The two lines also includes different values in EVENT_CLASS_CODES as 'PO_PA' and as 'PO_CO'.
Can someone please explain the reasons for these two lines or the front end screen business process activity that would have caused this to happen?

Thank you.