Components are mounted twice

### Version
- react-native-router-flux v4.0.6
- react v16.6.1
- react-native v0.57.5
- ios 12.x

### Expected behaviour
Navigate back to a component and navigate back again. Didmount should be executed once.

### Actual behaviour
Navigate to the component again immediately after a component back , it should not cause the component to be mounted twice .

### Steps to reproduce
Navigate from "home" to ”productList“
```javascript











```

### Reproducible Demo
The official demo can reproduce this bug

I think this may be because I re-entered the component immediately after exiting the component.

My English is not very good, I hope you can understand, God bless 🙏