SQL & PL/SQL

How do I display multiple rows data as one Row? And also eliminate any duplicates among them..

For Example

Column1              Column2

A0                       JAN, FEB, MAR, SEP

B0                       JAN, FEB

C0                       JUN, JUL, AUG, SEP, OCT, NOV, DEC

My Output should be

COLUMN2

JAN,FEB,MAR,SEP,JUN,JUL,AUG,OCT,NOV,DEC