Increment metric duration

header.go

@@ -70,6 +70,12 @@ func ParseHeader(input string) (*Header, error) {
 
 // NewMetric creates a new Metric and adds it to this header.
 func (h *Header) NewMetric(name string) *Metric {
+	for i := range h.Metrics {
+		if h.Metrics[i].Name == name {
+			return h.Metrics[i]
+		}
+	}
+
 	return h.Add(&Metric{Name: name})
 }
 

metric.go

@@ -81,7 +81,7 @@ func (m *Metric) Start() *Metric {
 func (m *Metric) Stop() *Metric {
 	// Only record if we have a start time set with Start()
 	if !m.startTime.IsZero() {
-		m.Duration = time.Since(m.startTime)
+		m.Duration += time.Since(m.startTime)
 	}
 
 	return m