RCOIS AddItem Behavior

Hi,

While adding a child product onto a Complex product (CxP) using RCOIS AddItem method (no UI context), if there are any Constraint rules OR compatibility rules OR cardinality rule defined in the CxP or in its class or in its parent's class, saying it cannot be added twice.

In what conditions we get the error in RCOIS AddItem method ?

And in what other cases AddItem removes the old instance of a child product & adds a new instance of the same product ?

Is this related to the fact that if child product is based on a class or not ? If yes, how ?