Thank you for your post.
Firstly, FileMaker never displays the license key over the network.
It is possible your license key was randomly made by another person. Also, although it hasn't happened previously, we could have issued the same license key twice (anything is possible).
If you are still having difficulty with the other customer, please contact me privately (top of this page - right side - just below the blue bar - X Messages) with your license key information, and I'll see if it has been duplicated or registered under another name.
One of my customers is having the same issue -- conflicts with misc unknown people on a hosted FM10 server. My client is using a license key that came with a new, shrink-wrapped 'academic' version of Pro 10. I am working with my client, FMS host, and my FileMaker SE to try to determine if it is in fact a duplicate key or if it might be a larger issue.