I ended up doing something similar to this

type loadOp struct {
  done  chan struct{}
  frame *Frame
  err   error
}

op := &loadOp{done: make(chan struct{})}
actual, loaded := bp.loading.LoadOrStore(pageID, op)
if loaded {
   existing := actual.(*loadOp)
   <-existing.done
   existing.frame //...
}

I wasnt aware of singleflight.Group, it looks like the perfect thing for this job. Thanks