Add PreserveDuplicateAttrs to ReadSettings

When an element has two or more attributes with the same
name, they are all preserved instead of leaving only one.
This setting is disabled by default.

etree.go

@@ -46,6 +46,10 @@ type ReadSettings struct {
 	// false.
 	PreserveCData bool
 
+	// When an element has two or more attributes with the same name,
+	// preserve them instead of keeping only one. Default: false.
+	PreserveDuplicateAttrs bool
+
 	// Entity to be passed to standard xml.Decoder. Default: nil.
 	Entity map[string]string
 }
@@ -836,7 +840,7 @@ func (e *Element) readFrom(ri io.Reader, settings ReadSettings) (n int64, err er
 		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)
+				e.createAttr(a.Name.Space, a.Name.Local, a.Value, e, settings.PreserveDuplicateAttrs)
 			}
 			stack.push(e)
 		case xml.EndElement:
@@ -1232,17 +1236,20 @@ 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)
+	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) *Attr {
-	for i, a := range e.Attr {
-		if space == a.Space && key == a.Key {
-			e.Attr[i].Value = value
-			return &e.Attr[i]
+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]
+			}
 		}
 	}
+
 	a := Attr{
 		Space:   space,
 		Key:     key,

etree_test.go

@@ -1394,3 +1394,72 @@ func TestReindexChildren(t *testing.T) {
 		}
 	}
 }
+
+func TestPreserveDuplicateAttrs(t *testing.T) {
+	s := `
+		<document attr="test" attr="test2"></document>
+	`
+	t.Run("Test PreserveDuplicateAttributes", func(t *testing.T) {
+		doc := NewDocument()
+
+		doc.ReadSettings = ReadSettings{
+			PreserveDuplicateAttrs: true,
+		}
+
+		err := doc.ReadFromString(s)
+		if err != nil {
+			t.Error("etree: unable to read document", err)
+		}
+
+		document := doc.FindElement("document")
+
+		if len(document.Attr) != 2 {
+			t.Error("etree: should have found 2 attributes")
+		}
+
+		if document.Attr[0].Value != "test" {
+			t.Errorf("etree: expected value test got %s", document.Attr[0].Value)
+		}
+
+		if document.Attr[0].Key != "attr" {
+			t.Errorf("etree: expected attribute key to be attr got %s", document.Attr[0].Key)
+		}
+
+		if document.Attr[1].Value != "test2" {
+			t.Errorf("etree: expected value test2 got %s", document.Attr[0].Value)
+		}
+
+		if document.Attr[1].Key != "attr" {
+			t.Errorf("etree: expected attribute key to be attr got %s", document.Attr[0].Key)
+		}
+
+	})
+
+	t.Run("Test Default Settings", func(t *testing.T) {
+		doc := NewDocument()
+
+		doc.ReadSettings = ReadSettings{
+			PreserveDuplicateAttrs: false,
+		}
+
+		err := doc.ReadFromString(s)
+		if err != nil {
+			t.Error("etree: unable to read document", err)
+		}
+
+		document := doc.FindElement("document")
+
+		if len(document.Attr) != 1 {
+			t.Error("etree: should have found 1 attribute")
+		}
+		if document.Attr[0].Value != "test2" {
+			t.Errorf("etree: expected value test got %s", document.Attr[0].Value)
+		}
+
+		if document.Attr[0].Key != "attr" {
+			t.Errorf("etree: expected attribute key to be attr got %s", document.Attr[0].Key)
+		}
+
+	})
+
+}