Clicking modify partitions while installer is searching for partitions causes disk to show up twice

**Describe the bug**
Clicking modify partitions while installer is searching for partitions causes disks to show up twice.


**To Reproduce**
Steps to reproduce the behavior:
1. Go to custom (advanced) setup
2. Click on modify partitions while installer is searching for partitions
3. Close gparted window
4. All disks should show up twice.

It's a bit tricky in a vm when searching for partitions takes <0.5 s, but on bare metal in my case this search takes ~15s and I was able to pull it off every time.

**Expected behavior**
Regardless of user's shenanigans each disk should show up only once.

**Desktop (please complete the following information):**
- OS: elementary OS 6 beta 2021.04.30