Unlike most other notifications, patrons do not get to define what format they receive overdue notices. So, if a library offers all formats, the patron will get all formats, provided they have the address/phone number needed. Why these notices are handled differently from other notices, I do not know, but it would be nice if patrons had a choice in how they get these notices. If it is a matter of them getting always getting a notice, perhaps one format should be on by default and not changeable unless they have the required elements for another delivery format. Having a global setting for patrons on overdues is not patron friendly.
Some libraries have to send the third or more notices by mail. They don't want to give patrons the choice intentionally, so we might need some configuration options for the messaging table. This would actually be nice in general, as for example the ILL module has a lot of notices now, but a library using ILL might not use all of them and need to hide some.