Rejecting redundant options in Create Collation

Hi,

Currently, it's not an error for CREATE COLLATION to be invoked
with options repeated several times. The last (rightmost) value is kept
and the others are lost.
For instance CREATE COLLATION x (lc_ctype='en_US.UTF8',
lc_collate='en_US.UTF8', lc_ctype='C')
silently ignores lc_ctype='en_US.UTF8'. But that kind of invocation
isn't likely to be legit. It's more plausible that it's the result of
some mistake or confusion.
The same goes for the other options:

CREATE COLLATION [ IF NOT EXISTS ] name (
[ LOCALE = locale, ]
[ LC_COLLATE = lc_collate, ]
[ LC_CTYPE = lc_ctype, ]
[ PROVIDER = provider, ]
[ DETERMINISTIC = boolean, ]
[ VERSION = version ]
)

I suggest the attached simple patch to raise an error when any of
these options is specified multiple times.

Best regards,
--
Daniel Vérité
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Twitter: @DanielVerite