I have just had to add several words to the user dictionary for the spell-checker in Notepad++, that I have already added to my user dictionary in LibreOffice, and to my user dictionary in Firefox (all under Windows 10 – does this happen with user dictionaries under Unix & Mac operating systems?).
Under the “DRY” (Don’t Repeat Yourself) principle, a user should not have to accept a word’s spelling more than once.
User dictionaries should not be in a “walled garden” within an application. They should exist at operating-system level, or more specifically, at user-account level.
Or, until Microsoft (and other operating system vendors) implement this, applications — at least, open source applications like those listed above — should make their user dictionaries accessible to each other.
Some issues to consider: users with dictionaries in more than one language; security.