I know a lady on the yahoo list that has both graves AND hashimoto's antibodies - so while it is unusual, it can happen at the same time.
Or while it may not be another thyroid issue, it could be another endocrine or or auto-immune issue. Too often, the doctors find one thing and think "eureka" when the body is going "hey, I am NOT finished".
Case in point, I had a pituitary tumor and some thyroid nodules and hashimoto's. The docs decided that the only problem I had was hypothyroid. Thought a couple pills and I would be golden. It just did not work out that way.
With complete testing, turns out I had PCOS, Cushing's, Prolactinoma, Hashimoto's, Vit D deficiency, Ferretin Deficiency, etc. etc. etc.
If the doc stops at the thyroid - other things can go, well, uncovered.