#126 print error if grok field is defined multiple times

exporter/grok.go

@@ -66,8 +66,11 @@ func verifyFieldName(metricName string, template template.Template, regex *onigu
 					return fmt.Errorf("%v: field name %v is ambigous, as this field is defined in the grok pattern but is also a global field provided by grok_exporter for the %v", metricName, grokFieldName, description)
 				}
 			} else {
-				if !regex.HasCaptureGroup(grokFieldName) {
+				numGroups := regex.NumberOfCaptureGroups(grokFieldName)
+				if numGroups == 0 {
 					return fmt.Errorf("%v: grok field %v not found in match pattern", metricName, grokFieldName)
+				} else if numGroups > 1 {
+					return fmt.Errorf("%v: grok field %v found %d times in match pattern: this is ambiguous, the pattern should define each grok field exactly once", metricName, grokFieldName, numGroups)
 				}
 			}
 		}

oniguruma/oniguruma.go

@@ -83,8 +83,15 @@ func (regex *Regex) Free() {
 }
 
 func (regex *Regex) HasCaptureGroup(name string) bool {
-	_, err := regex.getCaptureGroupNums(name)
-	return err == nil
+	return regex.NumberOfCaptureGroups(name) > 0
+}
+
+func (regex *Regex) NumberOfCaptureGroups(name string) int {
+	groups, err := regex.getCaptureGroupNums(name)
+	if err != nil {
+		return 0
+	}
+	return len(groups)
 }
 
 func (r *Regex) getCaptureGroupNums(name string) ([]C.int, error) {