Ensure templates have different destinations

nomad/structs/structs.go

@@ -2129,11 +2129,19 @@ func (t *Task) Validate(ephemeralDisk *EphemeralDisk) error {
 		}
 	}
 
+	destinations := make(map[string]int, len(t.Templates))
 	for idx, tmpl := range t.Templates {
 		if err := tmpl.Validate(); err != nil {
 			outer := fmt.Errorf("Template %d validation failed: %s", idx+1, err)
 			mErr.Errors = append(mErr.Errors, outer)
 		}
+
+		if other, ok := destinations[tmpl.DestPath]; ok {
+			outer := fmt.Errorf("Template %d has same destination as %d", idx+1, other)
+			mErr.Errors = append(mErr.Errors, outer)
+		} else {
+			destinations[tmpl.DestPath] = idx + 1
+		}
 	}
 
 	return mErr.ErrorOrNil()

nomad/structs/structs_test.go

@@ -526,6 +526,19 @@ func TestTask_Validate_Template(t *testing.T) {
 	if !strings.Contains(err.Error(), "Template 1 validation failed") {
 		t.Fatalf("err: %s", err)
 	}
+
+	// Have two templates that share the same destination
+	good := &Template{
+		SourcePath: "foo",
+		DestPath:   "local/foo",
+		ChangeMode: "noop",
+	}
+
+	task.Templates = []*Template{good, good}
+	err = task.Validate(ephemeralDisk)
+	if !strings.Contains(err.Error(), "same destination as") {
+		t.Fatalf("err: %s", err)
+	}
 }
 
 func TestTemplate_Validate(t *testing.T) {