Button command fired twice?

Hello,

I have a simple page with a button linked to a command:

<Button Text="ContactWise" Command="{Binding NavigateToPage}" CommandParameter="ContactWise" />

In the "NavigateToPage" command, there's nothing but this code:

Debug.WriteLine("Pressed button")

The problem is that I get that output:

Pressed button
Pressed button

The event seems to be fired twice! Why?