You’re right. I don’t know if this varies by state, but for sure I’ve had to sign contacts stating I didn’t have any other insurance in order to start various plans. So I don’t think this is universally ok but clearly there are situations that allow it, seemingly most common in conjunction with Medicare.
This is funny, because basically every insurance here in Sweden states that if you are covered by another insurance, then this one is invalid. In practice though what Iv'e heard is that the companies will always just be kind and sort of split the responsibility between them instead when something happens.
