Attributes deduplicated more efficiently

When reading an XML document, this package uses a more
time-efficient technique to detect and remove attributes
with duplicated names (within each element).

etree.go

@@ -889,6 +889,7 @@ func (e *Element) readFrom(ri io.Reader, settings ReadSettings) (n int64, err er
 		r = newXmlSimpleReader(ri)
 	}
 
+	attrCheck := make(map[string]int)
 	dec := newDecoder(r, settings)
 
 	var stack stack[*Element]
@@ -921,8 +922,21 @@ func (e *Element) readFrom(ri io.Reader, settings ReadSettings) (n int64, err er
 		switch t := t.(type) {
 		case xml.StartElement:
 			e := newElement(t.Name.Space, t.Name.Local, top)
-			for _, a := range t.Attr {
-				e.createAttr(a.Name.Space, a.Name.Local, a.Value, e, settings.PreserveDuplicateAttrs)
+			if settings.PreserveDuplicateAttrs {
+				for _, a := range t.Attr {
+					e.addAttr(a.Name.Space, a.Name.Local, a.Value)
+				}
+			} else {
+				for i, a := range t.Attr {
+					key := a.Name.Space + ":" + a.Name.Local
+					if j, contains := attrCheck[key]; contains {
+						e.Attr[j].Value = a.Value
+					} else {
+						e.addAttr(a.Name.Space, a.Name.Local, a.Value)
+						attrCheck[key] = i
+					}
+				}
+				clear(attrCheck)
 			}
 			stack.push(e)
 		case xml.EndElement:
@@ -1365,25 +1379,24 @@ func (e *Element) addChild(t Token) {
 // prefix followed by a colon.
 func (e *Element) CreateAttr(key, value string) *Attr {
 	space, skey := spaceDecompose(key)
-	return e.createAttr(space, skey, value, e, false)
-}
 
-// createAttr is a helper function that creates attributes.
-func (e *Element) createAttr(space, key, value string, parent *Element, preserveDups bool) *Attr {
-	if !preserveDups {
-		for i, a := range e.Attr {
-			if space == a.Space && key == a.Key {
-				e.Attr[i].Value = value
-				return &e.Attr[i]
-			}
+	for i, a := range e.Attr {
+		if space == a.Space && skey == a.Key {
+			e.Attr[i].Value = value
+			return &e.Attr[i]
 		}
 	}
 
+	return e.addAttr(space, skey, value)
+}
+
+// addAttr is a helper function that adds an attribute to an element.
+func (e *Element) addAttr(space, key, value string) *Attr {
 	a := Attr{
 		Space:   space,
 		Key:     key,
 		Value:   value,
-		element: parent,
+		element: e,
 	}
 	e.Attr = append(e.Attr, a)
 	return &e.Attr[len(e.Attr)-1]