Duplicate Reducer Method Invokation

Hey there! I'm reporting a bug that seems to have also appeared in `use-immer`.

When invoking a reducer function, it fires twice initially, but it appears that it does not modify the state twice.

I've reproduced it in CodeSandbox: https://codesandbox.io/s/immer-reducer-duplicate-invokation-iylwj

`use-immer` Issue: https://github.com/immerjs/use-immer/issues/11

The solution seemed to have something to do with the need to memo a `produce()`'d reducer, or something along those lines.

Thanks!