message-tags: forbid duplicate tags in CLIENTTAGDENY

[emersion]

Main motivation is to disallow the following case:

CLIENTTAGDENY=foo,-foo

[ArsenArsen]

in similar vein, could we forbid * anywhere but in the first list item? i.e. make CLIENTTAGDENY=-foo,* and CLIENTTAGDENY=foo,* invalid

[emersion]

While I'm confident that no existing server sends CLIENTTAGDENY=foo,-foo, I wouldn't say the same about * being first. Additionally, CLIENTTAGDENY=foo,-foo is nonsensical while * position would be mainly about simplifying parsing.

[ArsenArsen]

IMO it's not more sensible since, for any tag list T, and any tag list without star elements T2, T,*,T2is equivalent to *,T2

[ArsenArsen]

ah, nevermind, that clause exists already; I just missed it