Allow multiple merge keys

decode.go

@@ -772,7 +772,8 @@ func (d *decoder) mapping(n *Node, out reflect.Value) (good bool) {
 			ni := n.Content[i]
 			for j := i + 2; j < l; j += 2 {
 				nj := n.Content[j]
-				if ni.Kind == nj.Kind && ni.Value == nj.Value {
+				// Key collisions are allowed if both of the keys are merges - each merge will be applied in order, leading to no collisions in the final decoded document
+				if ni.Kind == nj.Kind && ni.Value == nj.Value && !isMerge(nj) {
 					d.terrors = append(d.terrors, fmt.Sprintf("line %d: mapping key %#v already defined at line %d", nj.Line, nj.Value, ni.Line))
 				}
 			}
@@ -816,7 +817,7 @@ func (d *decoder) mapping(n *Node, out reflect.Value) (good bool) {
 	mergedFields := d.mergedFields
 	d.mergedFields = nil
 
-	var mergeNode *Node
+	mergeNodes := []*Node{}
 
 	mapIsNew := false
 	if out.IsNil() {
@@ -825,7 +826,7 @@ func (d *decoder) mapping(n *Node, out reflect.Value) (good bool) {
 	}
 	for i := 0; i < l; i += 2 {
 		if isMerge(n.Content[i]) {
-			mergeNode = n.Content[i+1]
+			mergeNodes = append(mergeNodes, n.Content[i+1])
 			continue
 		}
 		k := reflect.New(kt).Elem()
@@ -852,8 +853,8 @@ func (d *decoder) mapping(n *Node, out reflect.Value) (good bool) {
 	}
 
 	d.mergedFields = mergedFields
-	if mergeNode != nil {
-		d.merge(n, mergeNode, out)
+	for _, node := range mergeNodes {
+		d.merge(n, node, out)
 	}
 
 	d.stringMapType = stringMapType

decode_test.go

@@ -1530,6 +1530,31 @@ func (s *S) TestMergeNestedStruct(c *C) {
 	c.Assert(testm["outer"], DeepEquals, wantm)
 }
 
+const doubleMerge = `
+one: &one
+  a: 1
+  b: 2
+
+two: &two
+  b: 3
+  c: 4
+
+merged:
+  <<: *one
+  <<: *two
+`
+
+func (s *S) TestDoubleMerge(c *C) {
+	var m map[string]interface{}
+	err := yaml.Unmarshal([]byte(doubleMerge), &m)
+	c.Assert(err, IsNil)
+	c.Assert(m["merged"], DeepEquals, map[string]interface{}{
+		"a": 1,
+		"b": 3,
+		"c": 4,
+	})
+}
+
 var unmarshalNullTests = []struct {
 	input              string
 	pristine, expected func() interface{}