Jans,
In this case 3 outputs are appearing. One is RD00 with Print medium and 2 ZD00 outputs. Out of these RD00 is giving error with red mark and one ZD00 is not processed and other ZD00 output processed and IDocs created.
After creating the billing document, do you see RD00 and two entries of ZD00 in the output screen? Getting the same output type twice is surprising to me. Please paste the screenshot.
You are getting RD00 automatically in the transaction document (billing doc) is because there is a valid condition record created for it (VV31, display VV33).
TW