Prevent adding duplicates to a list

In a list, if the item already exists there, prevent me from adding it twice.

Maybe grey-out the text or background colour?

See 'Shop-Shop'!!

I tend to add items to my shopping list as I think of them - maybe when I'm reaching for something in the cupboard, and it's not there...

Trouble is, in the course of a week, I might reach for that item 2 or 3 times and so add it to the list, 2 or 3 times!

(And don't tell me it's my age! ;-)