Warning: Each child in an array or iterator should have a unique "key" prop
_I am..._

| -------------------------------------------------------------------------------------------------
| Reporting a bug or issue
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**Expected behavior:**
Styleguide example builds without warnings or errors


**Observed behavior:**
Warnings start appearing from first run.
Console output:
```
console> Warning: Each child in an array or iterator should have a unique "key" prop. Check the render method of `Palette`. See https://fb.me/react-warning-keys for more information.
    in Swatch (created by Palette)
    in Palette (created by Document)
    in view (created by View)
    in View (created by Section)
    in view (created by View)
    in View (created by Section)
    in Section (created by Document)
    in view (created by View)
    in View (created by Document)
    in Document
⛏  Built ./main.js in 1733ms
```


**How to reproduce:**
Clone to desktop
Run **Styleguide** example 


**Sketch version:**
49.2
Node v9.11.1
npm 5.6.0


**Please attach screenshots, a zip file of your project, and/or a link to your github project**
![screen shot 2018-04-20 at 6 02 25 pm copy](https://user-images.githubusercontent.com/2784323/39079016-b34ae210-44c7-11e8-975c-6e50418f69be.png)
