follow up for #2809535: Convert AJAX part of \Drupal\system\Tests\Ajax\MultiFormTest to JavascriptTestBase
There is a version of assertNoDuplicateIds on AssertContentTrait but this is meant for Kernel tests. We need a version to use for functional tests.
The version here \Drupal\Tests\file\Functional\FileFieldDisplayTest::assertNoDuplicateIds might be a good start.
Existing implementations should then use the method on the trait