Previously copied element is created in the Hierarchy on selecting the "Paste" option in the StyleSheet window.

Steps to repro:
1. Create a new Project.
2. Window > UI Toolkit > UI Builder.
3. Library > Double click on Button.
4. Hierarchy > Button > Right-click > Duplicate.
5. StyleSheet > Right-click > Paste.

Actual Result:
Elements are created in the Hierarchy window.

Expected Result:
Elements should not be created in the Hierarchy window.

Reproducible in:
2021.1.0a10, 2020.2.0f1, 2020.1.1f1, 2019.4.0f1 with Package version 1.0.0-preview.9.

Environment:
Occurring on Windows 10 & Mac 10.15.