If you're referring to an actual leaf and spine fabric architecture, they have their own mechanisms to deal with multiple L2 paths.

If you're referring to classical L2 switch hardware, that happens to use a leaf and spline like architecture, it would need a STP to break L2 loops and could not leverage the advantages of an actual leaf and spine fabric architecture.

For example, in the Cisco world an actual leaf and spine fabric architecture might be provided by ACI.