Why the weak key is created as unique

mydb=> create table AAA (a serial primary key);
NOTICE: CREATE TABLE will create implicit sequence 'aaa_a_seq' for SERIAL column 'aaa.a'
NOTICE: CREATE TABLE/PRIMARY KEY will create implicit index 'aaa_pkey' for table 'aaa'
CREATE

mydb=> create table BBB (a serial references AAA, b integer, primary key(a,b));
NOTICE: CREATE TABLE will create implicit sequence 'bbb_a_seq' for SERIAL column 'bbb.a'
NOTICE: CREATE TABLE/PRIMARY KEY will create implicit index 'bbb_pkey' for table 'bbb'
NOTICE: CREATE TABLE/UNIQUE will create implicit index 'bbb_a_key' for table 'bbb'
NOTICE: CREATE TABLE will create implicit trigger(s) for FOREIGN KEY check(s)
CREATE

mydb=> insert into AAA values (1);
INSERT 20369 1

mydb=> insert into BBB values (1,1);
INSERT 20370 1
mydb=> insert into BBB values (1,2);
ERROR: Cannot insert a duplicate key into unique index bbb_a_key

I would like that the pair keys (a,b) was unique.
Certainly, I can remove unique index 'bbb_a_key'...
But how more correctly?
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Marat Khairullin mailto:xmm(at)rambler(dot)ru
Marat(dot)Khairullin(at)f92(dot)n5049(dot)z2(dot)fidonet(dot)org
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