pgsql: Fix duplicate primary keys in partitions

Fix duplicate primary keys in partitions

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

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ff347f8aff04865680c19ffc818460bb2afaad5b

Modified Files
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src/backend/catalog/index.c            | 10 +++++-----
src/test/regress/expected/indexing.out | 18 +++++++++++++++++-
src/test/regress/sql/indexing.sql      |  9 ++++++++-
3 files changed, 30 insertions(+), 7 deletions(-)