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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 attributes attribute is expected to be handled in that fashion in any context.
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