According to No.10's ePetition site "The most common reason for rejection is duplication - many users have commented that there are many petitions on similar subjects clogging up the site and we are trying to eliminate too much duplication or overlap, though balancing that with the need to allow for the nuances for similar petitions which take the subject in a slightly different direction." (Click here for more.)
Obviously this sort of manipulation by the webmaster must be checked in a fully functional Interactive Democracy. Maybe checking would be the responsibility of an inter-party Parliamentary committee.
Using the search facility makes it easy to identify ePetitions with a similar theme. These should be filed in a root directory and assessed as a group by Parliament, who would then present the choices in a referendum.