avoiding duplicated shares

A short proposal on simplifying the issue we have with duplicating sharing data. Often, a new team member shares with me sub folders or files in folders that are already shared. So they get duplicated and how up in my root. My sync client then syncs them, and well, I rather not have a copy of our Launch Marketing folder synced twice as that will definitely fill my hard drive.

If somebody shares a file with somebody else but that person has already access, we should just give a short notification (NO modal) saying "user already has access to the file", and do nothing. This fixes a scenario like "I share folder X with person A and B, Person A shares subfolder folder Y from folder X with person B, B now has stuff double".

This doesn't fix a scenario like "I share folder X with person A, Person A shares sub folder folder Y from folder X with person B, I then share Folder X with person B" - and there are other troublesome issues like when permissions don't match.

We can't fix everything in one go, but we could fix the most common mistake with the change I propose and that would already save a ton of mess.

@jancborchardt @nimishavijay as you asked

Let me link this to the issue of handling shares in a smarter way https://github.com/nextcloud/server/issues/39148

Start with the linked issue #39148 and trace the existing file-sharing flow for the case where the recipient already has access. The proposed behavior is a notification without a modal and no duplicate share; completion should cover the described folder and file scenario while leaving the acknowledged permission mismatches out of scope.