[master branch] Theme switching in Control Catalog is broken

[maxkatz6]

Describe the bug
Application crashes, when user switches theme.

To Reproduce
Steps to reproduce the behavior:

1. Open control catalog
2. Select any other theme in the themes drop down
3. Wait couple of seconds
4. See error

Desktop (please complete the following information):

• OS: Windows, Mac (I assume any other OS affected too)
• Version: master branch 8bede23

Additional context

Probably introduced by #5070
IS NOT fixed with this PR - #5686

An item with the same key has already been added. Key: Avalonia.Animation.DoubleTransition
   at System.ThrowHelper.ThrowAddingDuplicateWithKeyArgumentException[T](T key)
   at System.Collections.Generic.Dictionary`2.TryInsert(TKey key, TValue value, InsertionBehavior behavior)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Avalonia.Animation.Animatable.AddTransitions(IList items) in line 189