Clicking the submit button is one submission. Double-clicking it is still one submission. So, I don't think that would be the issue.
However, sendmail could be the culprit.
If you can Telnet or SSH into your server, run a command called top.
It shows you the current processes and CPU load. The processes that are using the most CPU are shown at the top.
A lightly loaded server will have a load average of less than 1. A load average greater than, say, 3 or 4 is OK but starting to get busy. A load average of 20 or more means the server is going to be very unresponsive.
To test your theory, run top and do some quick form submissions. If Sendmail is using a lot of your CPU you'll see it straightaway.
If you're using a shared hosting service, your hosting provider should be able to track down the problem for you.
If not, you might consider hiring an expert on RH to diagnose the problem.