Return equal number of rows with same column value

[Nick <nboutelier@xxxxxxxxx>]

For the data...

INSERT INTO test (id,val) VALUES (1,a);
INSERT INTO test (id,val) VALUES (2,a);
INSERT INTO test (id,val) VALUES (3,a);
INSERT INTO test (id,val) VALUES (4,a);
INSERT INTO test (id,val) VALUES (5,b);
INSERT INTO test (id,val) VALUES (6,b);

How could I return an even amount of val? For example, I would like to
return this...

1 | a
2 | a
5 | b
6 | b

Since the least number of b vals is 2, Id like to limit the a columns
to return only 2

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