Hi -- set a rule on the field bound to the non-repeating text box. The rule would have the action to populate the value in a field, and you'd select the field in the repeating table that you want the value populated to. Then, for the value, select the fx button and select your initial field that is bound to the non-repeating text box. I've attached a simple sample. Right click the link in the header of this post and save the file locally. Then right click the file and open in design mode. Fill out the non repeating text box, and the rows below will populate.
You could also set the repeating field to have a default value of the non repeating field... instead of using rules....