SQL: pull distinct values form 1 column with all values from 2nd column
Col 1,       Col 2
1,            a
1,            b
2,            a
2,            b
2,            c

1,a,b
2,a,b,c

UPDATE t
SET    t.dupcustodians = dt.custadmin
FROM   tbldoc t
       INNER JOIN (SELECT t1._dupid,
                          (SELECT DISTINCT custadmin + ', '
                           FROM   tbldoc t2
                           WHERE  t2._dupid = t1._dupid
                           ORDER  BY custadmin + ', '
                           FOR XML PATH('')) AS custadmin
                   FROM   tbldoc t1
                   GROUP  BY _dupid) AS dt
         ON t._dupid = dt._dupid
            ;

SELECT Col1, GROUP_CONCAT(Col2)
FROM MyTable
GROUP BY Col1