Browser going to page twice?

I'm having a weird problem. When I submit a form on my site, it often sends twice. I'm not sure if this is a client-side or server-side problem, but it doesn't happen on other sites. Is this a common problem, or am I making some dumb mistake? -- The above message is encrypted with double rot13 encoding. Any unauthorized attempt to decrypt it will be prosecuted to the full extent of the law.