Validation Summary duplicates error messages

I have a problem that I've been trying to solve for quite at while now... I have a user control containing both client and server side validation. When clicking the submit button, that causes validation, the client side validation fires perfectly well. A neat error message is shown for each client side validator. I fill out the missing client side validated fields and hit the submit button again, without filling out the server side validated controls. A new summary shows up, but each server side validation error message is repeated TWICE! I can't find a solution to how to only get ONE message per validation control. I've tried to add additional Page.Validate() to see if perhaps more instances of the error message would show up. But still, two messages per control. I also created a new project and tested this using only an ASPX and an ASCX file. This works as it should! My "real" solution is quite big and comprises of eight projects. We also use something that could be ...