What you're asking for is a deep comparison before watch re-executes to identify if its dependencies have truly changed or did an ABA (they changed A -> B, e.g. set.clear(), but then went back to inital state B -> A, e.g. set.add(item)).

Note that watch already performs this check, although:

• it's a shallow test: it looks at set instance, not set contents.
• it's skipped when using { deep: true }, otherwise your watch would never run.
• (source: https://github.com/vuejs/core/blob/main/packages/reactivity/src/watch.ts#L244-L248)

It's a tough thing to ask for a deep check as it requires watch() to keep a deep copy of the dependencies, for the purpose of having something to compare to.
How deep should this copy be? Why stop at one level? What if there are cycles? etc.

In your specific example, I think it would just be more efficient for you to check if set and model.value have the same elements before doing the mutation in your first watch.