Hi @jdbrngr, thanks for reaching out to the Community!
You can definitely filter your back in stock flow as you would any other Klaviyo flow. For your purposes, add a flow filter to your flow that states what someone has done > placed order at least once over all time > where collection > Collection A > at least once > over all time.
Excluding customers who already bought a product from that collection would be more complicated and I wouldn’t necessarily recommend it. While in most cases avoiding a duplicate email would be ideal, with the Back in Stock Flow customers opt-in to be notified on a per item basis. If they really like a popular item of yours, they might opt-in more than once if that item sells out regularly, and they want to receive that message.
I hope this helps!