Duplicates can appear in project list

![image](https://user-images.githubusercontent.com/2193314/38326810-a0f94cc4-37fb-11e8-8993-9516b86274f1.png)

This is on Windows with the following settings:

```
  "gitProjectManager.baseProjectsFolders": [
    "$HOME/dev",
    "%USERPROFILE%\\Documents\\dev"
  ],
  "gitProjectManager.maxDepthRecursion": 3,
```