If you have a recipe that uses 150g flour in step 1, then later uses 350g flour in step 4, then you actually have two separate ingredients:
1. 150g plain flour
2. 350g plain flour
It may useful to present an ingredient summary for the purposes of shopping which collects that to 500g flour, but from when trying to represent the recipe, it's actually two different ingredients, and it'd be a false re-use to try to re-use the node or try to treat those ingredients as a single ingredient of greater quantity.
Just because two things appear otherwise identical, doesn't mean they are.
I sometimes see a similar issue in software development and re-use of classes or functions. Two different cases happen to need identical handling, so get put through the same code paths. Then later it turns out one case needs to be treated slightly differently, so that path ends up with special casing, and you get a mess of special handling with case statements or "bool treatDifferently" parameters.
If things are conceptually different, then treat them differently, don't assume false commonality.