According to :h spell
Vim only checks words for spelling, there is no grammar check.
It does however alert with a green wobbly line if a sentence begins with a small letter. Is there a way to have vim alert in a similar way if a word is repeated? This is a mistake I make quite often.
It should be fairly easy to find with a regular expression, e.g. with \v( \w{2,})\1\W
. But how to apply this to vim's spellchecking?