Catch duplicate attributes

Author: hildjj

examples/xml.peggy

@@ -28,6 +28,17 @@ function clumpStrings(vals) {
   }
   return ret
 }
+
+function convertAttr(attr, error) {
+  const ret = {}
+  for (const {name, value, loc} of attr) {
+    if (ret[name]) {
+      error(`Duplicate attribute "${name}"`, loc)
+    }
+    ret[name] = value
+  }
+  return ret
+}
 }}
 {
   const names = []
@@ -173,13 +184,14 @@ element
 STag = '<' name:pushName attr:(S @Attribute)* S? '>' { return {
   type: 'element',
   name,
-  attr,
+  attr: convertAttr(attr, error),
 }}
 
 Attribute = name:Name Eq value:AttValue { return {
   type: 'attribute',
   name,
   value,
+  loc: location(),
 }}
 ETag = '</' popName S? '>'
 
@@ -210,7 +222,7 @@ content = c1:CharData? children:((element / Reference / CDSect / PI / Comment) C
 EmptyElemTag = '<' name:Name attr:(S @Attribute)* S? '/>' { return {
   type: 'element',
   name,
-  attr,
+  attr: convertAttr(attr, error),
 }}
 
 // Elements in the DTD