Can a line manager play dual role as product owner as well (dual role)?
Most of the scrum team is reporting (as line) to the product owner. How do you see this? is this realistic to have?
I'd suggest that there is nothing very realistic about providing reports at all. Valuable products ought to be delivered instead. These provide real evidence of progress. Anyone who is a line manager -- whoever they are -- ought to use their influence and authority to expedite this process.
They can - yet should they? There isn't a simple answer, and in part may depend on the person trying to fulfill both.
Product Owner is an accountability in Scrum, not a role or a job title. In some cases, a line manager could take on the accountabilities of a Product Owner within the Scrum Team. They may even have a good balance of authority needed which so many proxy Product Owners are missing today. However, the Product Owner accountabilities listed in the Scrum Guide are the minimum needed for Scrum to be effective. A Product Owner in my experience will need many other Product Management skills to be effective. It usually takes a full-time effort to be a successful product Owner.
The downside is that there could be conflicts of duty for one person to hold both. Wearing too much of the line manager hat may impact the Developer's ability to self-manage.
Another downside may be an impact on trust and psychological safety. Will people feel safe when admitting a mistake or take risks knowing that the Product Owner is also the person who could impact their merit review?