I normally add items to my shopping list and mark them done, without removing them. This helps me keep a list of what I’ve previously needed as a reminder.
When add items, I have to either scroll through the todo list and find if what I’m adding is already there, or just add a duplicate item.
A possible solution could be to automatically filter the todo list when adding an item, possibly with a toggle as it might be counterproductive for some use cases. The shown items could then be unchecked, or I could continue filling in the needed item.