Having primary key in both fields but repeated in either fields allowed repeating
You can achieve the same goal by having a UNIQUE composite index on the desired fields.

ALTER TABLE YourTable
  add CONSTRAINT YourTable_unique UNIQUE (A, B,....);


(A, B,...) - fields A, B,... should be declared as NOT NULL.

This might be helpful if you have a surrogate PRIMARY KEY which you may want to keep. Something similar was recently asked for MySQL here, but the principle is the same.

You can also perform the same operation during the CREATE TABLE statement:

CREATE TABLE table_name
(
  column1 datatype NOT NULL,
  column2 datatype NOT NULL,
  ...

  CONSTRAINT constraint_name UNIQUE (uc_col1, uc_col2, ... uc_col_n)
);