I have recently encountered a presentation about scrum where stakeholders and users were visualized as equal roles like Development Team, Scrum Master and Product Owner. Is this valid in terms of Scrum? I assume that the framework might be adjusted but this looks redundant.
In Scrum, there are only three roles - Scrum Master, Product Owner, and Development Team member. Scrum does recognize stakeholders - the Scrum Guide talks about them in several contexts, such as the Scrum Master working with stakeholders to help them understand how Scrum works and how to best interact with the Scrum Team and stakeholder involvement in the Sprint Review. Stakeholders, however, aren't specific roles in the context of the Scrum Team.
The primary interface between the external stakeholders and the Scrum Team is the Product Owner. The Product Owner takes the considerations of the external stakeholders into account when expressing and ordering Product Backlog Items as well as when ensuring the visibility and understanding of the state of the Product Backlog.
The Product Owner doesn't have to be a gate between external stakeholders and the Development Team. Depending on the organization, it may be appropriate for the Development Team to interact directly with external stakeholders. The Scrum Master can help facilitate and maximize the effectiveness of these interactions.
Users ARE stakeholders
A stakeholders, by definition, is anyone who "holds" a "stake" in the project/product. And the term is broad, meaning also anyone who is affected by the project/product.
Some stakeholders are more important than others. They are called Key Stakeholders, and might be the project sponsor for example, or a product manager that decides on what gets built, as opposed to users who might just offer suggestions or have to use what's given to them. The Product Owner deals with stakeholder needs when deciding what the development team should build.
So you can visualize users and stakeholders as the same group. But again, within that group, different stakeholders might have different powers over the outcome of the project/product.