Panic on duplicate command names (#317)

build.go

@@ -158,6 +158,11 @@ MAIN:
 		}
 	}
 
+	// Validate if there are no duplicate names
+	if err := checkDuplicateNames(node, v); err != nil {
+		return nil, err
+	}
+
 	// "Unsee" flags.
 	for _, flag := range node.Flags {
 		delete(seenFlags, "--"+flag.Name)
@@ -311,3 +316,20 @@ func buildGroupForKey(k *Kong, key string) *Group {
 		Title: key,
 	}
 }
+
+func checkDuplicateNames(node *Node, v reflect.Value) error {
+	seenNames := make(map[string]struct{})
+	for _, node := range node.Children {
+		if _, ok := seenNames[node.Name]; ok {
+			name := v.Type().Name()
+			if name == "" {
+				name = "<anonymous struct>"
+			}
+			return fmt.Errorf("duplicate command name %q in command %q", node.Name, name)
+		}
+
+		seenNames[node.Name] = struct{}{}
+	}
+
+	return nil
+}

kong_test.go

@@ -1662,3 +1662,35 @@ func TestMapDecoderHelpfulErrorMsg(t *testing.T) {
 		})
 	}
 }
+
+func TestDuplicateName(t *testing.T) {
+	var cli struct {
+		DupA struct{} `cmd:"" name:"duplicate"`
+		DupB struct{} `cmd:"" name:"duplicate"`
+	}
+	_, err := kong.New(&cli)
+	assert.Error(t, err)
+}
+
+func TestDuplicateChildName(t *testing.T) {
+	var cli struct {
+		A struct {
+			DupA struct{} `cmd:"" name:"duplicate"`
+			DupB struct{} `cmd:"" name:"duplicate"`
+		} `cmd:""`
+		B struct{} `cmd:""`
+	}
+	_, err := kong.New(&cli)
+	assert.Error(t, err)
+}
+
+func TestChildNameCanBeDuplicated(t *testing.T) {
+	var cli struct {
+		A struct {
+			A struct{} `cmd:"" name:"duplicateA"`
+			B struct{} `cmd:"" name:"duplicateB"`
+		} `cmd:"" name:"duplicateA"`
+		B struct{} `cmd:"" name:"duplicateB"`
+	}
+	mustNew(t, &cli)
+}