The previously suggested approach prevents double clicking the Next button at my end. Here's a screencast
1. First the default behavior, which causes navigation towards two steps to occur: e.g. if you start at Step1 and double-click Next the Wizard navigates to Step2 and then to Step3
2. The modified (by the logic in the OnClientButtonClicking event handler) behavior, which on double-click causes navigation to the next step only, instead of passing through 2 steps: e.g. starting from Step1 a double-click only navigates to Step2.
Attached you can find the sample runnable page used to record the screencast. Could you give it a try and let us know in case you notice a different behavior than the one demonstrated in the linked screencast?