Add Duplicate Variant error

Author: eemeli

spec/errors.md

@@ -178,6 +178,27 @@ appears on the left-hand side of more than one _option_ in the same _expression_
 > {{This is {$foo}}}
 > ```
 
+### Duplicate Variant
+
+A **_<dfn>Duplicate Variant</dfn>_** error occurs when the same _keys_
+are used for more than one _variant_.
+
+> Examples of invalid messages resulting in a _Duplicate Variant_ error:
+>
+> ```
+> .match {$var :string}
+> * {{The first default}}
+> * {{The second default}}
+> ```
+>
+> ```
+> .match {$x :string} {$y :string}
+> *   foo   {{The first "foo" variant}}
+> bar *     {{The "bar" variant}}
+> *   |foo| {{The second "foo" variant}}
+> *   *     {{The default variant}}
+> ```
+
 ## Resolution Errors
 
 **_<dfn>Resolution Errors</dfn>_** occur when the runtime value of a part of a message

spec/syntax.md

@@ -369,6 +369,8 @@ satisfied:
 - At least one _variant_ MUST exist whose _keys_ are all equal to the "catch-all" key `*`.
 - Each _selector_ MUST have an _annotation_,
   or contain a _variable_ that directly or indirectly references a _declaration_ with an _annotation_.
+- Every _variant_ MUST NOT use the same set of _keys_ in the same order as another _variant_.
+  _Literal_ _keys_ are compared by their contents, not their syntactical appearance.
 
 ```abnf
 matcher         = match-statement 1*([s] variant)

test/schemas/v0/tests.schema.json

@@ -342,6 +342,7 @@
               "missing-selector-annotation",
               "duplicate-declaration",
               "duplicate-option-name",
+              "duplicate-variant",
               "unresolved-variable",
               "unknown-function",
               "unsupported-expression",

test/tests/data-model-errors.json

@@ -164,6 +164,22 @@
           "type": "duplicate-option-name"
         }
       ]
+    },
+    {
+      "src": ".match {$var :string} * {{The first default}} * {{The second default}}",
+      "expErrors": [
+        {
+          "type": "duplicate-variant"
+        }
+      ]
+    },
+    {
+      "src": ".match {$x :string} {$y :string} * foo {{The first foo variant}} bar * {{The bar variant}} * |foo| {{The second foo variant}} * * {{The default variant}}",
+      "expErrors": [
+        {
+          "type": "duplicate-variant"
+        }
+      ]
     }
   ]
 }