Parameters: fix for dup parameters after writing updated params to AC v3.2

Parameter collection received from vehicle after writing updating parameters can contain duplicates w/ AC v3.2 (e.g. FS_THR_ENABLE below). Fixed by adding simple uniqueness check when populating parameter adapter.
![screenshot_2014-07-15-00-21-52](https://cloud.githubusercontent.com/assets/4014433/3580599/760221aa-0bd9-11e4-9f15-1f9b2b5d6987.png)