License allocated to multiple people

I purchased a license renewal for Norton Antivirus last month and activated my three PC's with this OK.  Last week it started failing on two of the machines saying that the license was not valid.  I tried entering the key again and the response was that it is allocated to someone else.

The initial machine I used it on is still working OK so this raises a few questions.

1.  How is it possible for someone else to not only be allocated but actually use my license key ?

2.  Are duplicate keys being issued by Norton ?

3. If they are not being issued by Norton has some got access to their key database and is fraudulantly using them ?

I have spoken to support about this and so far they have been no help whatsoever.  Has anyone else had a simlar problem with their license being hijacked and how to fix the error ?