I'm not quite sure what you are talking about. a set = singular noun, so use the singular. a user and a customer = two people, so use plural.

In many situations, the writer of statements like these does not know how many people he or she is discussing. There might be one user or customer, but there might be more. In these situations, there is a convention to use "(s)" at the end of appropriate words in the sentence in order to convey both a singular and plural situation. Your sentence is actually a way to write two sentences as one sentence. Here are the two sentences: The user and customer are often not the same person. The users and customers are often not the same persons (or people). The use of three (s)'s conveys that the writer wrote both of the above sentences, even though he did not.