BUG #14890: Error grouping by same column twice using FDW

The following bug has been logged on the website:

Bug reference: 14890
Logged by: Sean Johnston
Email address: sean(dot)johnston(at)edgeintelligence(dot)com
PostgreSQL version: 10.0
Operating system: Linux Ubuntu
Description:

Using user 'sean' with database 'sean':

create extension postgres_fdw;
create table tbl (col1 integer);
create server loop foreign data wrapper postgres_fdw options ( host
'localhost', dbname 'sean' );
create user mapping for sean server loop options ( user 'sean' );
create foreign table loop_tbl (col1 integer) server loop options (
table_name 'tbl' );

select col1, col1 from loop_tbl group by 1, 2;
psql:t.sql:7: ERROR: targetlist item has multiple sortgroupref labels