Summary:
**Test plan**
1. Add fonts using `react-native link` and a `"rnpm"` config in package.json
2. Manually delete one or more fonts from Xcode's Resources folder using Remove References option
3. Run `react-native link` again
With the patch in this PR, the plist will not accidentally get populated with duplicate entries.
Closes https://github.com/facebook/react-native/pull/12048
Differential Revision: D4560432
fbshipit-source-id: aba3733acfd5373f8d654406b06b8338b137bc07