When using the CREATE TABLE .. PARTITION OF syntax, it's possible to
cause a partition to get two primary keys if the parent already has one.
Tighten the check to disallow that.

Reported-by: Rajkumar Raghuwanshi
Author: Amul Sul
Discussion: https://postgr.es/m/CAKcux6=OnSV3-qd8Gb6W=KPPwcCz6Fe_O_MQYjTa24__Xn8XxA@mail.gmail.com