Observed an issue in CREATE TABLE syntax

I observed an issue that even if invalid syntax is provided for CREATE TABLE, table is getting created successfully.

Below table creation succeed even though same constraint name is given multiple times.
None of the below constraints has any meaning of giving multiple times.

postgres=# create table new (id int NULL NULL);
CREATE TABLE

postgres=# create table new1(id serial NOT NULL NOT NULL);
CREATE TABLE

postgres=# create table new2 (id int unique unique);
CREATE TABLE

Should we not throw error for above syntaxes?

Please find the attached patch with the fix.

Thanks and Regards,
Kumar Rajeev Rastogi