InfoPath Dev

I have a custom workflow I set up that kicks off based on values of hidden checkboxes in the form. Once the check box is set (true/yes), the next step of the workflow starts.  The workflow is set to start when an item is created or changed. 

So the last step is to send an email to the employee when the last checkbox is checked.  That's the only condition - if hidden checkbox is checked - email user.   The problem is, it sends the same email twice, immediately.  So I instantaneously get the same email twice when testing. 

This does not appear to be an infrastructure/exchange setup problem, because I've set up a test email on another doc library, and it works fine (one single email sent).

Anyone experience this before?

I guess I should test all available options before posting here. 

Instead of having an additional step for the notification email, I just made it part of the previous step.  I added an action before the email - "Wait for [checkbox] to equal Yes".   Then send the email. 

Now it only sends one email.  However, I have no idea why.  What would be the difference between waiting for a value to change, or a condition on that value?