Check if the entity is already referenced by the field doesn't work with taxonomy

working on this issue at DrupalCon Nashville

I'll take a lool too to familiarise in Drupalcon Nashville

I checked the original function and it has this code now:

if ($value['entity'] && $value['entity']->id() == $entity->id()) {
  $form_state->setError($reference_form['entity_id'], t('The selected @label has already been added.', ['@label' => $labels['singular']]));
  break;
}

It was changed here:
#2675684: Existing reference can not be replaced with another

I have a fix in comment #12 on #3011469: Inline Entity Form won't submit after form error, that also has test coverage.

Copying patch over from #3011469: Inline Entity Form won't submit after form error

Since it has test coverage, I think it's an improvement on #2.

It also has test only patch to demonstrate the bug.

Reroll, #9 wouldn't apply against current HEAD.

Rerolling

Oops, empty patch.

This was fixed with #3011469: Inline Entity Form won't submit after form error