There's a known issue with the version 6.4.4-2 of the Wacom driver. Intermittent "extra" clicks can be occur if you're using a pen in an application. On the Wacom site you can find the following workaround:
What you can do about it while we fix it: If your product is fully supported in Wacom Center: go to the pen settings page, click Advanced, and set Double-tap distance to "Off".
For all other products: go to the Pen tab in Wacom Tablet Properties and set Tip Double Click Distance to "Off".
Another try is to go back to a previous version of the Wacom driver.