State tax filing

"Did you have Illinois-source income after you moved to California?" I did not, but my employer didn't update my work location so officially both my w2 and IL revenue department think I did. Both IL and CA are taxing this income. Therefore, I think I am due tax credit from CA.

I cannot obtain a refund from IL without my employer providing the letter. Are you saying there's nothing I can do about being double-taxed?