Can Product Owner play dual roles

shouldn't be ...let's see what experts have to say on this

There is nothing in the Scrum Guide that says it isn't possible, so technically yes it is possible.  I have seen it done for the Scrum Master role.  The key will be that this one person can be 100% committed to the Product Owner role.  Think carefully about what the outcomes may be for one person having to wear both hats, though.  And be sure to ask the Development Team their thoughts on such an arrangement.

I think:

1 - PO is a member of a Dev Team  - absolutelly ok

2 - PO is also a Scrum Master - not good. I do not want to sat that it is impossible but in my mind this is not right

3 - PO is a member of another Dev Team or SM - in general not good but if you can - it is ok

4 - PO is also working in another project (waterfall) - again, a little bit strange, but if you can it is ok 

The Scrum Guide says:

Fewer than three Development Team members decrease interaction and results in smaller productivity gains

<...>

The Product Owner and Scrum Master roles are not included in this count unless they are also executing the work of the Sprint Backlog.

Therefore the PO and Scrum Master can also be Development Team members.

There is no restriction that PO cannot play a dual role however I see only one challenge that at any one point  he / she will be responsible and accountable for one area.

e.g

Sprint planning

• During What he can play a role of PO and discusses the objective
• During How activity he will play the role of Developer

Daily Scrum - Dual responsibility (Major role as Developer)

• Role of developer however at some point when requirement are not clear he will/ should play role of PO as well

Sprint Review - Dual responsibility (Major role as a PO)

• As a Product Owner as he or she should invite the stakeholder and explain product backlog item that has been done
• As a development team to discuss what went well during the Sprint what problems it ran into, and how those problems were solved

Role of developer however at some point when requirement are not clear he will/ should play role of PO as well

Unsure how a requirement can be "unclear" when the PO is embedded within the Development Team, or why such clarification would be needed during the Daily Scrum.

I know that the Scrum Guide leaves open the possibility of either the Scrum Master or the Product Owner to be a part of the Development Team, but I just see many issues impacting both team maturity and proper Scrum with such a dynamic.