I have made some recipes and put them in a book, but sometimes when a book is spawned or when its contents are updated, this happens:

The recipe should show an iron ingot, 2 gunpowder, and 3 gravel, but it displays duplicates of the ingredients that are used more than once in the recipe.

Can you provide the recipe that you used for this? You mention it sometimes happens, have you found any repeatable way to cause it to happen?

This is the recipe:

combine Shotgun Shell
gravel + gravel + gravel + sulphur + iron_ingot + sulphur
= sulphur
@name §6Shotgun Shell

It seems to happen when a book is updated or when a book is spawned after running /rmreloadbooks or /rmreload.