unique node counting

[Flians]

Hi, when I define egg::CostFunction, I want to count unique nodes.
But I met the following problem:
For (* (+ a b) (+ a b)), using AstSize will output that the node size is 3. But the results of two (+ a b) are the same and should be from the same node. So, I hope the node size is 2. Could you please tell me how to achieve it? Thank you.