Insurance covered procedure then decided they didn't cover procedure and wants money back?

BCBS told me that they DID cover a certain procedure ( not pre-existing malady ) and so I went forward with procedure. They paid the claim. I then had same procedure done a second time and submitted 2nd claim. BCBS said 'oh, we don't cover that......... I said 'yes you do, you told me that you did and you covered it 3 months ago'....... they said 'whoops, mistake, give us our money back'. Can they do that ? Thieves. Totally. And I asked them to look into it and investigate the issue ........they said they would and would get back with me. They never called me back on it. Then I reiceve a collection letter in the mail from some 3rd party angency demanding the $1000. How is this legal ? Any SERIOUS offerings of knowledge on this would be much appreciated ! thx