There is no legal definition of MAU. However the industry standard is that it represents total number of customers, so it wouldn't make sense to count one person twice if they have two accounts, because they are still just one customer.
Tangential, but twitter's customers aren't their users, it's their advertisers. The users are the product.
Feels like a quirk comes from the fact that a lot of prominent pseudonymous Twitterers, that they make accounts per communities and go extra miles to dis-link each ones, especially from real life identities.
often they are agents that manage social media for different businesses. they should be counted as separate
They are still one set of eyeballs and one brain to advertise to. Since the metric is mainly for the purpose of "how many eyeballs might see these ads" it still makes more sense to count them as one.