Voynich Decoded

[tavie]

Why do we never see ff, tt, kk, or pp doubled? You can't have two planets together - no Saturn Saturn, no Mars Mars.

From a mere cosmetic point of view, gallows glyphs would look awful written together.  All of them have a rightside loop that would clash into the next gallows.  I'm not saying that is the reason (although I do have a headcanon that the scribes's glyph choice may have at least partially been influenced by avoiding clashes) since there are other glyphs that aren't commonly duplicated that could easily be so in theory.  

And that takes me onto a wider point:  Voynichese really doesn't like duplicating glyphs. It's only common for two glyphs.  Probably it's less likely duplicates occur anyway, since positional rigidity means some are always initials and some are always finals.  But there's nothing obvious stopping /chch/ or /aa/ and those are relatively uncommon.

So I'd be cautious about relying on that for your planet identifications.