Denied - Vote has already been registered for device (Submission Vote Identity 2.7)

So I have two user names.

I innocently used the second user name and clicked on a news article up vote and got: Denied - Vote has already been registered for device (Submission Vote Identity 2.7)

So. I use linux, Firefox and have browser cookies set to auto delete when the browser closes. I also have a city wide IP address. So it is not on my IP address.

So. I closed the browser and checked cookies. Zero cookies are listed. So how does Voat know and give Denied - Vote has already been registered for device (Submission Vote Identity 2.7)