why am i getting an error in my sql code?

my code: CREATE INDEX classdescription_idx ON class(classdescription);

my error:

Error starting at line : 36 in command -

CREATE INDEX classdescription_idx ON class(classdescription)

Error at Command Line : 36 Column : 57

Error report -

SQL Error: ORA-01408: such column list already indexed

01408. 00000 - "such column list already indexed" *Cause: *Action: