Clarify that Duplicate Option Name is a data model error (#577)

* Clarify that Duplicate Option Name is a data model error

* Apply suggestions from code review

Co-authored-by: Addison Phillips <[email protected]>

* Update spec/syntax.md

Good callout

Co-authored-by: Eemeli Aro <[email protected]>

---------

Co-authored-by: Addison Phillips <[email protected]>

Author: eemeli

spec/formatting.md

@@ -245,8 +245,6 @@ The result of resolving _option_ values is a mapping of string identifiers to va
 For each _option_:
 
 - Resolve the _identifier_ of the _option_.
-- If the _option_'s _identifier_ already exists in the resolved mapping of _options_,
-   emit a Duplicate Option Name error.
 - If the _option_'s right-hand side successfully resolves to a value,
    bind the _identifier_ of the _option_ to the resolved value in the mapping.
 - Otherwise, bind the _identifier_ of the _option_ to an unresolved value in the mapping.

spec/syntax.md

@@ -89,7 +89,8 @@ Attempting to parse a _message_ that is not _well-formed_ will result in a _Synt
 
 A _message_ is **_<dfn>valid</dfn>_** if it is _well-formed_ and
 **also** meets the additional content restrictions
-and semantic requirements about its structure defined below.
+and semantic requirements about its structure defined below for
+_declarations_, _matcher_ and _options_.
 Attempting to parse a _message_ that is not _valid_ will result in a _Data Model Error_.
 
 ## The Message
@@ -535,7 +536,10 @@ optional whitespace.
 The value of an _option_ can be either a _literal_ or a _variable_.
 
 Multiple _options_ are permitted in an _annotation_.
-Each _option_ is separated by whitespace.
+_Options_ are separated from the preceding _function_ _identifier_
+and from each other by whitespace.
+Each _option_'s _identifier_ MUST be unique within the _annotation_:
+an _annotation_ with duplicate _option_ _identifiers_ is not valid.
 
 ```abnf
 option = identifier [s] "=" [s] (literal / variable)