Verify if the double printing occurs if you try printing from another system.
Also does the double print occur when you perform a printer config print from the printer panel.
If the issue occurs from another server and the panel printing you’ve got either a firmware or mechanical failure on the device. contact the manufacture.
If the issue is only occurring when printing from the AS/400 please post your printer configuration for review.
Also here’s a system i printing tech site I’ve referred to in the past.