Yes, depending on the union contract, your States laws and the ploicy of the municipality. You should contact the union or the attorney handling the workers compensation claim. I am not a WA attorney, but the employer has the right to verify the injury and the rehabilitation of the injury through their own experts.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
I agree with the previous answer , and would ask to you want to back to work and put this behind you? They are just getting a second opinion and if there is a re-injury this will come up. There may be a union issue, and also talk with your doctor about the second opinion.
Your employer can require you to attend what's called an Independent Medical Exam. Employers usually request these exams in order to obtain a second opinion. Usually, the exams occur before the attending physician issues a release to full duty work, so it's somewhat unclear to me why this seems to be coming up now. One of the previous answers suggested that it might be in case this ever comes up again. For example, if you later feel that your injury related condition is worse, the employer and Department will want some medical opinions stating exactly what your condition was when you were released to return to full duty work.
Disclaimer: This answer does not consitute legal advice and no attorney/client relationship is formed.