fix(parser): make attribute re-declaration a warning

Technically a strict XML fail. However due to the way we expose
attributes via a lazy getter it may be hard for library users to
properly deal with that error (as it is being logged out-of-bounds).

parser.js

@@ -307,6 +307,7 @@ function Saxen(options) {
           alias,
           name,
           attrs = {},
+          seenAttrs = {},
           w,
           j;
 
@@ -385,11 +386,14 @@ function Saxen(options) {
         // advance cursor to next attribute
         i = j + 1;
 
-        if (attrs[name]) {
-          handleError('attribute <' + name + '> already defined');
+        // check attribute re-declaration
+        if (name in seenAttrs) {
+          handleWarning('attribute <' + name + '> already defined');
           return cachedAttrs = false;
         }
 
+        seenAttrs[name] = true;
+
         if (!isNamespace) {
           attrs[name] = value;
           continue;

test/elements.js

@@ -979,7 +979,7 @@ test({
   xml: '<e:root xmlns:e="http://extensions" xmlns:e="http://other" />',
   ns: true,
   expect: [
-    ['error', 'attribute <xmlns:e> already defined'],
+    ['warn', 'attribute <xmlns:e> already defined'],
     ['openTag', 'e:root', false],
     ['closeTag', 'e:root']
   ],
@@ -990,18 +990,40 @@ test({
   xml: '<root xmlns="http://extensions" xmlns="http://other" />',
   ns: true,
   expect: [
-    ['error', 'attribute <xmlns> already defined'],
+    ['warn', 'attribute <xmlns> already defined'],
     ['openTag', 'ns0:root', false],
     ['closeTag', 'ns0:root']
   ],
 });
 
-// local attribute re-declaration / default namespace
+// local attribute re-declaration / no ns
 test({
   xml: '<root a="A" a="B" />',
   expect: [
-    ['error', 'attribute <a> already defined'],
+    ['warn', 'attribute <a> already defined'],
     ['openTag', 'root', false],
     ['closeTag', 'root']
   ],
-});
+});
+
+// local attribute re-declaration / with namespace
+test({
+  xml: '<root xmlns="http://extensions" a="A" a="B" />',
+  ns: true,
+  expect: [
+    ['warn', 'attribute <a> already defined'],
+    ['openTag', 'ns0:root', false],
+    ['closeTag', 'ns0:root']
+  ],
+});
+
+// local attribute re-declaration / with other namespace
+test({
+  xml: '<root xmlns="http://extensions" xmlns:bar="http://extensions" bar:a="A" bar:a="B" />',
+  ns: true,
+  expect: [
+    ['warn', 'attribute <bar:a> already defined'],
+    ['openTag', 'ns0:root', false],
+    ['closeTag', 'ns0:root']
+  ],
+});