Changes have been made in Git for this ticket:

Some DBs require field names in ORDER BY to appear in SELECT list (Bug 
#10383).

  3 files changed, 5 insertions(+), 1 deletions(-)
http://git.horde.org/horde-git/-/commit/277daa7c3ccaaac2a1b6a1b770879bd9278a5975

I had a quick look into it. The following patch resolves the problem 
(at least for me).

--- turba/lib/Driver/Sql.php    2011-08-03 15:39:05.242893493 +0200
+++ /tmp/10383.php      2011-08-03 15:38:56.482904733 +0200
@@ -257,7 +257,7 @@
              $nameFormat = 'first_last';
          }

-        $order = $this->_buildFields($fields);
+        //$order = $this->_buildFields($fields);
          $joins = $this->_buildJoin($fields);
          $where = $this->_buildWhere($fields);

@@ -274,7 +274,8 @@
                               $this->map['__owner'],
                               $this->map['__owner'],
                               $where[$i],
-                             $order[$i]);
+                             $this->map['__key']);
+                             //$order[$i]);

              /* Run query. */
              try {

/turba/search.php?search_mode=duplicate

2011-07-26T09:47:25+02:00 ERR: HORDE [turba] SQL QUERY FAILED: 
SQLSTATE[42P10]: Invalid column reference: 7 ERROR:  for SELECT 
DISTINCT, ORDER BY expressions must appear in select list
LINE 1: ...IS NOT NULL AND a1.object_lastname <> '' ORDER BY a1.object_...
                                                              ^
         SELECT DISTINCT a1.object_id FROM turba_objects a1 JOIN turba_objects
           a2 ON a1.object_lastname = a2.object_lastname AND 
a1.object_firstname
           = a2.object_firstname AND a1.object_id <> a2.object_id WHERE
           a1.owner_id = 'thpo' AND a2.owner_id = 'thpo' AND a1.object_lastname
           IS NOT NULL AND a1.object_lastname <> '' ORDER BY
           a1.object_lastname,a1.object_firstname [pid 9137 on line 
808 of "/usr/share/php/Horde/Db/Adapter/Base.php"]