Steps to reproduce:
1. Create and open a new Unity project
2. In the toolbar press "GameObject" > "UI" > "Event System"
3. Drag the "EventSystem" game object from the Hierarchy window to the Project window
4. Drag the created prefab from the Project window into the scene view

Expected result: A warning is thrown stating that you can't use multiple Event Systems, only one Event System gets placed in the Scene view
Actual result: No warning is thrown stating that you can't use multiple Event Systems, multiple Event Systems get placed in the Scene view

Reproducible with: 2017.4.36f1, 2018.4.15f1, 2019.2.18f1, 2019.3.0f4, 2020.1.0a19