Are intersecting ways possible?

Hm I see. Thanks for the answer. I think in my use case, I am only interested in roads so if the roads are guaranteed to be unique that is fine. Can you explain if that is the case?

I don't know of any specific examples, but I know there are places in the world that have elevated highways with other roads underneath, so you might also need to consider the layer tag. Apart from that I can't immediately think of why two different roads would contain the same two consecutive nodes.

Stacked highways should be mapped separately with separate nodes in my opinion. But of course my opinion does not rule out that such roads have been mapped differently.

I guess there will be a significant amount of road "loops" where one road separates from another roads only to re-join it shortly after (e.g. a separate lane at a bus stop). In that case two consecutive nodes of one way will be shared with one other way (on that way the nodes should not be consecutive, though).

Sometimes highways are mapped not as line features but as areas. That is often done for pedestrian areas but also for other roads. There are different ways of tagging but such area could just be tagged with highway=xyz and area=yes. Such areas may be joined to other road areas or line roads on their boarders.

There are some highway features (not highways in a strict sense) like highway_rest_area or highway_emergency_bay that could again overlap with real roads or one and other.

And of course you may be faced with mapping errors where for example an angled intersection is not mapped properly but the two intersecting roads share a short way.