-The previous platform added as a new key element when a new platform is selected in the dropdown in AdaptivePerformanceGeneralSettings.
-When adding the same platform again ArgumentException error is thrown in the console.

Steps to repro:
1. Create a new project.
2. Edit > Project Settings > Adaptive Performance > Install.
3. Project > Select AdaptivePerformanceGeneralSettings.
4. Inspector > Keys > Element 0 > Change to iOS.
5. Hover the mouse over Project Settings Window.

Actual Result:
The previous platform added as a new key element.

Expected Result:
The previous platform should not be added as a new key element.

Reproducible in:
2020.2.0a16, 2020.1.0b14, 2020.1.0a11 with Package Version 2.0.0-preview.8.

Note: Adaptive Performance and simulator option is not available in Project settings with Package Version 2.0.0-preview.4.

Environment:
Occurring on Windows 10 & Mac 10.15.

Stacktrace:
ArgumentException: An item with the same key has already been added. Key: iOS
System.Collections.Generic.Dictionary`2[TKey,TValue].TryInsert (TKey key, TValue value, System.Collections.Generic.InsertionBehavior behavior) (at <fb001e01371b4adca20013e0ac763896>:0)
System.Collections.Generic.Dictionary`2[TKey,TValue].Add (TKey key, TValue value) (at <fb001e01371b4adca20013e0ac763896>:0)
UnityEditor.AdaptivePerformance.Editor.AdaptivePerformanceGeneralSettingsPerBuildTarget.OnAfterDeserialize () (at library/packagecache/com.unity.adaptiveperformance@2.0.0-preview.8/editor/management/adaptiveperformancegeneralsettingsperbuildtarget.cs:122)
UnityEditor.PopupCallbackInfo:SetEnumValueDelegate(Object, String[], Int32)