The as-path prepending functionality implemented in bgpcep is missing the feature to add more than one instance of own AS number defined in section section 5.1.2 of RFC4271:
Whenever the modification of the AS_PATH attribute calls for including or prepending the AS number of the local system, the local system MAY include/prepend more than one instance of its own AS number in the AS_PATH attribute. This is controlled via local configuration.
Currently it is possible to prepend only one instance of own AS to the path.