Rocket Validator looks for accesibility violations on your pages using the axe-core accessibility engine, hosted on our own servers and integrated into our web crawler. You don't need to buy an additional license, everything's already included in your Pro account at Rocket Validator.
A11Y check: ID attribute value must be unique
Accessibility rule: The value assigned to the ID attribute on focusable elements must be unique to prevent the second instance from being overlooked by assistive technology. Put another way, ID attributes may not be used more than once on focusable elements in the same document in order to differentiate each focusable element from another.