CLDR-16251 Add constraint on duplicate variant tags

[macchiati]

CLDR-16251

• This PR completes the ticket.

ALLOW_MANY_COMMITS=true

[pedberg-icu]

Going ahead and merging this...

[gibson042]

Suggested change

	> As is often the case, the complete syntactic constraints are not easily captured by ABNF, so there is a further condition:
	> The sequence of variant subtags must not have any duplicates (eg, de-1996-fonipa-1996 is not syntactically well-formed).
	As is often the case, the complete syntactic constraints are not easily captured by ABNF, so there is a further condition:
	The sequence of variant subtags must not have any duplicates (eg, de-1996-fonipa-1996 is not syntactically well-formed).

(and for consistency with Unicode Locale Identifier, this should probably precede the grammar table)

[gibson042]

This should probably be merged with the comment above the table, e.g.

-As is often the case, the complete syntactic constraints are not easily captured by ABNF, so there is a further condition: There cannot be more than one extension with the same singleton (-a-, …, -t-, -u-, …). Note that the private use extension (-x-) must come after all other extensions.
+As is often the case, the complete syntactic constraints are not easily captured by ABNF, so there are further conditions:
+There cannot be more than one extension with the same singleton (-a-, …, -t-, -u-, …), and
+the sequence of variant subtags in a tlang must not have any duplicates.
+Note that the private use extension (-x-) must come after all other extensions, and can be followed by any subtags.

[gibson042]

@macchiati Note the above comments, which were submitted just after merge and therefore weren't addressed.

[pedberg-icu]

@macchiati Note the above comments, which were submitted just after merge and therefore weren't addressed.

@gibson042 @macchiati I filed https://unicode-org.atlassian.net/browse/CLDR-17211 to address those post-merge comments.