Opening an XML resource that is already open duplicates the contents

[keith-hall]

If you open an XML resource that is already open (from a sublime-package file, but not yet saved as an override), it correctly switches to the open tab, but it duplicates the contents of the file.

Example with Batch File/Comments.tmPreferences:

1. Preferences -> Browse Packages
2. Ensure there is no Batch File folder (i.e. delete it if there is one)
3. Switch back to ST3
4. Open Command Palette
5. Type PRV: O and select Package Resource Viewer: Open Resource
6. Select Batch File
7. Select Comments.tmPreferences
8. Repeat the previous step. The XML is duplicated each time.

Tested on ST3 builds 3114 and 3118 on Linux and Windows.