code: function checkNoSameAddressInDifferentParents(){
  		if (objUnit.parent_units.length === 1)
  			return callback();
  		var assocAuthors = {};
  		var found_address;
  		async.eachSeries(
  			objUnit.parent_units,
  			function(parent_unit, cb){
  				storage.readUnitAuthors(conn, parent_unit, function(arrAuthors){
  					arrAuthors.forEach(function(address){
  						if (assocAuthors[address])
  							found_address = address;
  						assocAuthors[address] = true;
  					});
  					cb(found_address);
  				});
  			},
  			function(){
  				if (found_address)
  					return callback("some addresses found more than once in parents, e.g. "+found_address);
  				return callback();
  			}
  		);
  	}
docs: avoid merging the obvious nonserials