"Create a new purchase order with the same item and distributor as … ", followed by some words to identify which existing purchase order you mean.
“that already existed in another purchase order” seems too vague for this instruction. How would the other person know which one you meant?
Your capitalisation of “item” and “Distributor” is inconsistent. Normally I dislike capitalisation of common nouns just because they “seem important”, but in “business-ese” cases like this it is not actually forbidden. But, you should choose one style and stick with it.