So, given this situation, do you have any suggestions on how to
approach this and reduce the amount of duplicated code as much as
possible?
It's not as powerful as \quoteDuring, but you could use variables to hold the unison parts: (I've used \relative here but it might be more useful to use \fixed in this situation.)
%%%
\version "2.24.4"
unison = { e4 f g e }
flute = \relative {
c''4 d e c |
\unison
d4 e f d
}
clarinet = \relative {
e''4 d c e
\unison
f4 e d f
}
\score {
<<
\new Staff \flute
\new Staff \clarinet
>>
}
%%%