This module works beyond the original use-case of adding product line items to an order. We use it anywhere Entity Browser would be considered overkill. Unfortunately, Inline entity form has it hard coded to not allow the same existing entity to be added twice.
This was added in this commit https://git.drupalcode.org/project/inline_entity_form/-/commit/087b72bf3... and #1526084: Abilty to add existing entities →
1. Add an existing entity using the add existing entity button.
2. Attempt to add the same entity a second time.
Notice the second time a form validation check blocks you.
Remove the hard coded bits that block a user from adding the same entity multiple times and return that control to the field widget configuration.
Discuss & implement.
This will add the UI back from core that allows a user to enable the same entity to be added multiple times.
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Active
2.0
Code
I've added a Merge Request to the fork that adds a new 'Allow users to re-use existing @label multiple times' checkbox, which, when checked, will skip the duplicate entity id validation and permit multiple re-use of existing entities.