Do not allow multiple flaggings when Field Entity Form is selected as Link Type

Problem/Motivation

Do not allow multi flagging when Flag Entry Form is selected as a Link type

Steps to reproduce

Create a new Flag with the Link type set to Field Entry Form. Go to node that is allowed to be flagged and copy the link of the "flag/bookmark", should be something like: /flag/details/flag/{flag_machine_name}/{node_id} so if your flag name is bookmark and you're node is is 1, the link is then /flag/details/flag/bookmark/1
For easier testing and distinction add a field to your Flag.
You could add multiple flags to the same node with the same user. Results can be seen with the default "my bookmarks" view.
Bug discovered in 📌 Editing/Removing a specific flagging is not possible when you have multiple flaggings on the same entity Active .

Proposed resolution

Check if entity is already flagged and instead of adding a new flag, provide the edit of the existing one.