I'm guessing you are using XLSForm to build your form. As far as I know, the only way to require at least one repeat is to edit the form XML after conversion to remove jr:template="" from your repeat.
I wrote a bit more about this at Side slider bar not able to be moved manually in long pick-list of ODK Collect - #7 by LN
I'm starting to think we should look into a modification for XLSForm such that jr:template is only added if defaults are specified for group elements. I'm not sure whether this would be difficult or not.