The installer gets confused when mount point assignment is used in webui, mount point mappings are configured and then one mapping is removed and configured again. A video will demonstrate it.
Steps to Reproduce:
1. Run a webui installation on a system with pre-existing partitioning (in my case it was a default installation of Fedora 38 Workstation in UEFI mode)
2. Select "Mount point assignment", create mappings for /boot, /, /boot/efi, /home
3. Remove the mapping for /boot/efi
4. Create a new mapping for /boot/efi
webui reports "Duplicate mount point." and the device previously selected for /home is de-selected. Next attempts to set the correct device for /boot/efi and /home mount points are not successful, since the selected device is set for both mount points.
Able to add a mapping for /boot/efi
Created attachment 1983635 [details]
I think it's probably worth considering this for a Final blocker (assuming we go ahead with WebUI), it's a close call for me whether it really *is* one.
Let's also consider this for Beta FE status...
+3 in https://pagure.io/fedora-qa/blocker-review/issue/1177 , marking accepted Beta FE.
Discussed during the 2023-08-28 blocker review meeting: 
The decision to classify this bug as an "AcceptedBlocker (Final)" was made as it violates the following criterion:
"The installer must be able to create and install to any workable partition layout using any file system and/or container format combination offered in a default installer configuration", read as applying to webUI's mount point assignment interface, in the case you need to re-do a mount point.
FEDORA-2023-7b30b63db9 has been submitted as an update to Fedora 39. https://bodhi.fedoraproject.org/updates/FEDORA-2023-7b30b63db9
FEDORA-2023-7b30b63db9 has been pushed to the Fedora 39 testing repository.
Soon you'll be able to install the update with the following command:
`sudo dnf upgrade --enablerepo=updates-testing --refresh --advisory=FEDORA-2023-7b30b63db9`
You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2023-7b30b63db9
See also https://fedoraproject.org/wiki/QA:Updates_Testing for more information on how to test updates.
I think this is resolved. I can't test exactly the same way because /boot/efi is now required on UEFI installs, but I tested with mounts for /var and /home and it seemed to behave correctly.
FEDORA-2023-7b30b63db9 has been pushed to the Fedora 39 stable repository.
If problem still persists, please make note of it in this bug report.