Multiple pages are created when using makeSymbol after saving
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| Reporting a bug or issue
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**Observed behavior:**

When using symbols, while running `npm run render`, every time I save any new changes, a new page is created in sketch. This doesn't happen if makeSymbol is not used, but if makeSymbol was ever used, even if it's removed, it keeps happening unless stopping process and running it again.

**How to reproduce:**

Run any example that uses makeSymbol like the one @jongold created [here](https://github.com/airbnb/react-sketchapp/blob/master/examples/symbols/src/my-command.js), make any change while running live reloading 

**Sketch version:** 

48.2

**Please attach screenshots, a zip file of your project, and/or a link to your github project**

Empty Project
![image](https://user-images.githubusercontent.com/5973652/36243329-0aa00e7a-11ef-11e8-9da9-2a5f8eea96d7.png)

After running `npm run render`, Page 2 is created

![image](https://user-images.githubusercontent.com/5973652/36243441-87087f06-11ef-11e8-9fd7-469a45e2e762.png)


After saving, Page 3 is created

![image](https://user-images.githubusercontent.com/5973652/36243367-34478244-11ef-11e8-80bf-159931ed8fad.png)

Is this how is supposed to work?
