I can not find a way to have both a natural and a flat accidental in front of a single note.
How can I get this in LilyPond?
Music: Practice & Theory Stack Exchange is a question and answer site for musicians, students, and enthusiasts. It only takes a minute to sign up.Sign up to join this community
This might be what you are looking for:
\tweak Accidental.restore-first ##t
As Ramillies says in the comments, this is a pretty rare occurrence; I'd be curious to see the context of your example. But the main reason I can think of these accidentals happening is to cancel out a prior double accidental (with the natural) while still giving a single accidental to the new pitch.
But LilyPond has various accidental styles that automates how accidentals appear in your scores. Depending on your specific example, you may find that one of these styles fits your needs the best.