Exclude Duplicate ID

Hi

Is there a way to exclude items from the feed if it has already created an item.

For example a product with ID has 3 variables S M and L so is there a way to stop it duplicating an item as currently it puts all 3 items in the feed.

I know I can use the only include default variation option but with thousands of items it would be time consuming to set a default in admin for all products individually.

Is there a rule option I might apply to say if item is created ignore items with same ID?

Many thanks
Ian