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Attributes designed for searching, such as givenName, sn, or mail, are often not handled correctly if multiple values are supplied in a federated context. So in general, no, one can't assume that (and it may be necessary to release an alternative single-valued attribute to some services), but it is a good practice to report such bugs when they are identified. In terms of correctness, any multi-valued attribute is expected to be handled in that fashion in any context.
Why does eduPerson include the eduPersonOrcid attribute but not eduPersonResearcherId? Won't this lead to new attributes for every kind of identifier?
Yes, it will, very deliberately. Combining multiple types of data into a single attribute precludes use cases in which only a subset of that data may be relevant, unless the data is encoded in a way that allows the different types of data to be recovered. That in turn adds extra work to a consumer of the data.
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