You probably just need to add a GROUP BY statement like:
SELECT dbmADI.*, GROUP_CONCAT(dbmSite.dbmSite ORDER BY dbmSite.dbmSite ASC SEPARATOR ', ') AS SelSites FROM dbmADI LEFT OUTER JOIN MRT07 ON dbmADI.dbmADIID = MRT07.MRT07dbmADIID LEFT OUTER JOIN dbmSite ON dbmSite.dbmSiteID = MRT07.MRT07dbmSiteID WHERE dbmADI.dbmADIID = ? GROUP BY dbmADI.dbmADIID