This is just a bug: there are no type params to substitute in the double-instantiated instance. The underlying is the same as the first instance.
https://go.dev/play/p/6WDvAzt01tY

It seems we are just missing a check that there are no type arguments.

May I propose a PR or a Plan on this?

Hi,
It looks like the double instantiation check was removed in CL 333589 when the instantiation logic was moved from subst.go to instantiate.go.

Current state:

 case *Named:
+    // Verify type parameter count for named types
+    tparams := orig.TypeParams()
+    if !check.validateTArgLen(pos, orig.obj.Name(), tparams.Len(), len(targs)) {
+        return Typ[Invalid]
+    }
+    if tparams.Len() == 0 {
+        // No type parameters to substitute (orig is not generic or already instantiated)
+        return orig  // nothing to do
+    }
     res = check.newNamedInstance(pos, orig, targs, expanding) // substituted lazily

This should restore the original behavior and prevent double instantiation. Happy to work on a CL if this approach looks acceptable.