In sequence on opening Search Index Manager window from Windows->Search then Multiple Search Index Manager windows get opened. Refer attached video.

Steps to repro:
1. Create a new project
2. Window > Search > Search Index Manager
3. Repeat step 2 multiple times.

Actual Result:
Multiple Search Index Manager windows get opened.

Expected Result:
On opening the Search Index Manager window multiple times it should only point to a single instance of that window.

Occurring on:
2021.2.0a6, 2021.2.0a1, 2021.1.0b8, 2021.1.0a1

Note:
Quick Search is a Built-In Package in the Unity 2021.1.0a1

Environment:
Occurring on Windows 10 & macOS 10.15