why am i getting an error in my sql code?

my code:

CREATE TABLE class (

Classcode char (4),

Class_description varchar (20) UNIQUE NOT NULL,

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: