CREATE COLLATION - check for duplicate options and error out if found one

Hi,

While reviewing [1], I found that the CREATE COLLATION doesn't throw an
error if duplicate options are specified, see [2] for testing a few cases
on HEAD. This may end up accepting some of the weird cases, see [2]. It's
against other option checking code in the server where the duplicate option
is detected and an error thrown if found one.  Attached a patch doing that.
I chose to have the error message "option \"%s\" specified more than once"
and parser_errposition because that's kind of agreed in [3].

Thoughts?

[1]
https://www.postgresql.org/message-id/CALj2ACWVd%3D-E6uG5AdHD0MvHY6e4mVzkapT%3DvLDnJJseGjaJLQ%40mail.gmail.com

[2]
CREATE COLLATION coll_dup_chk (LC_COLLATE = "POSIX", LC_COLLATE =
"NONSENSE", LC_CTYPE = "POSIX"); -- ERROR
CREATE COLLATION coll_dup_chk (LC_COLLATE = "NONSENSE", LC_COLLATE =
"POSIX", LC_CTYPE = "POSIX"); -- OK but it's weird
CREATE COLLATION coll_dup_chk (LC_CTYPE = "POSIX", LC_CTYPE = "NONSENSE",
LC_COLLATE = "POSIX"); -- ERROR
CREATE COLLATION coll_dup_chk (LC_CTYPE = "NONSENSE", LC_CTYPE = "POSIX",
LC_COLLATE = "POSIX",); -- OK but it's weird
CREATE COLLATION coll_dup_chk (PROVIDER = icu, PROVIDER = NONSENSE,
LC_COLLATE = "POSIX", LC_CTYPE = "POSIX"); -- ERROR
CREATE COLLATION coll_dup_chk (PROVIDER = NONSENSE, PROVIDER = icu,
LC_COLLATE = "POSIX", LC_CTYPE = "POSIX"); -- OK but it's weird
CREATE COLLATION case_sensitive (LOCALE = '', LOCALE = "NONSENSE"); -- ERROR
CREATE COLLATION coll_dup_chk (LOCALE = "NONSENSE", LOCALE = ''); -- OK but
it's weird
CREATE COLLATION coll_dup_chk (DETERMINISTIC = TRUE, DETERMINISTIC =
NONSENSE, LOCALE = ''); -- ERROR
CREATE COLLATION coll_dup_chk (DETERMINISTIC = NONSENSE, DETERMINISTIC =
TRUE, LOCALE = ''); -- OK but it's weird

[3]
https://www.postgresql.org/message-id/CALj2ACUa%3DZM8QtOLPCHc7%3DWgFrx9P6-AgKQs8cmKLvNCvu7arQ%40mail.gmail.com

With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com