[Bug]: `oc_share` table has duplicate rows for the same share

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

There might be a race condition in ShareProvider, where several requests in parallel check if share exists in DB, and create it otherwise. In this case, there could be several new entries created, each with unique id, but all pointing to the same share.
If user tries to modify/leave share, it will modify the first row only, keeping the second intact. That way, you can never get rid of it.

Steps to reproduce

Don't have clear steps, but example behaviour sounds logical:

1. Sharing a file in Talk creates a parent entry with share_type 10 and file_target /{TALK_PLACEHOLDER}/file.md
2. User access conversation with shared file as last message, filesystem is mounted from conversations request and chat messages request
3. This creates two entries in DB with share_type 11 and file_target /Talk/file.md (best reproducible when PHP debugger was enabled, but also occurs on prod/daily instances)
   3.1. Worse case if user at some point changed the attachments folder, so initial entry is share_type 11 and file_target /Talk/file.md, and duplicates are share_type 11 and file_target /SomeOtherPath/file.md. Modifications will touch only first occurence of /SomeOtherPath/file.md

Expected behavior

Some sort of transactional lock (to write in DB only once) in place

Nextcloud Server version

master

Start by tracing ShareProvider operations that check for an existing share and insert into the oc_share table. Reproduce the parallel-request scenario if possible, then determine how duplicate rows can be prevented transactionally. Done means concurrent creation no longer leaves duplicate entries for the same share, with coverage for the race if the project’s existing tests support it.