pg can create duplicated index without any errors even warnning

postgres=# create table t (a int, b int);
CREATE TABLE
postgres=# create index m on t(a);
CREATE INDEX
postgres=# create index m2 on t(a);
CREATE INDEX
postgres=# \d t
                 Table "demo.t"
 Column |  Type   | Collation | Nullable | Default
--------+---------+-----------+----------+---------
 a      | integer |           |          |
 b      | integer |           |          |
Indexes:
    "m" btree (a)
    "m2" btree (a)