The reason the TextChanged event fires twice in WP7 is a side effect of how the TextBox has been templated for the Metro look.
If you edit the TextBox template in Blend you will see that it contains a secondary TextBox for the disabled/read-only state. This causes, as a side effect, the event to fire twice.
You can change the template to remove the extra TextBox (and associated states) if you don’t need these states, or modify the template to achieve a different look in the disabled/read-only state, without using a secondary TextBox.
With that, the event will fire only once.