PL/SQL :: Rank Function And Duplicate Records

I have the following table :

CREATE TABLE A_TEST (A INTEGER, B INTEGER, C INTEGER, D INTEGER, FLAG CHAR(11));

INSERT INTO A_TEST (A,B,C,D) VALUES(1,2,3,4);
INSERT INTO A_TEST (A,B,C,D) VALUES(2,4,5,8);
INSERT INTO A_TEST (A,B,C,D) VALUES(1,2,3,4);
INSERT INTO A_TEST (A,B,C,D) VALUES(2,4,5,8);
[code].......

I would like to perform an update on the FLAG column by setting to "D" if it is a duplicate record.1,2,3,4);

I would like to use the rank function.

Desired update:
A     B     C     D     FLAG
1     2     3     4     
2     4     5     8     
1     2     3     4     D
2     4     5     8     D
7     2     3     4     
9     2     3     4     
7     2     3     4     D
1     2     3     4     D
5     4     5     8     
2     2     3     9     
2     4     5     8     
6     2     3     4     
1     3     3     4     
8     2     8     4