Ensure mute time intervals are unique in config, add associated test

Signed-off-by: Ben Ridley <benridley29@gmail.com>

config/config.go

@@ -442,6 +442,9 @@ func (c *Config) UnmarshalYAML(unmarshal func(interface{}) error) error {
 
 	tiNames := make(map[string]struct{})
 	for _, mt := range c.MuteTimeIntervals {
+		if _, ok := tiNames[mt.Name]; ok {
+			return fmt.Errorf("mute time interval %q is not unique", mt.Name)
+		}
 		tiNames[mt.Name] = struct{}{}
 	}
 	return checkTimeInterval(c.Route, tiNames)

config/config_test.go

@@ -210,6 +210,44 @@ route:
 
 }
 
+func TestMuteTimeNoDuplicates(t *testing.T) {
+	in := `
+mute_time_intervals:
+- name: duplicate
+  time_intervals:
+  - times:
+     - start_time: '09:00'
+       end_time: '17:00'
+- name: duplicate
+  time_intervals:
+  - times:
+     - start_time: '10:00'
+       end_time: '14:00'
+
+receivers:
+- name: 'team-X-mails'
+
+route:
+  receiver: 'team-X-mails'
+  routes:
+  -  match:
+      severity: critical
+     mute_times:
+     - business_hours
+`
+	_, err := Load(in)
+
+	expected := "mute time interval \"duplicate\" is not unique"
+
+	if err == nil {
+		t.Fatalf("no error returned, expected:\n%q", expected)
+	}
+	if err.Error() != expected {
+		t.Errorf("\nexpected:\n%q\ngot:\n%q", expected, err.Error())
+	}
+
+}
+
 func TestGroupByHasNoDuplicatedLabels(t *testing.T) {
 	in := `
 route: