update to detect duplicate DTSTART/DTEND in same ics event,

README.md

@@ -137,6 +137,9 @@ END:VCALENDAR
 `, function(err, data) { console.log(data); });
 ```
 
+note: If you using the ical.async.xxx functions is a separate async method from your mainline
+any thrown errors will be posted to that separate function.. so try/catch must be used around the ical.async.xxx functions
+
 ### autodetect
 These are the old API examples, which still work and will be converted to the new API automatically.
 Functions with callbacks provided will also have better performance over the older versions even if they use the old API.

ical.js

@@ -372,7 +372,7 @@ const exdateParameter = function (name) {
         if (typeof exdate[name].toISOString === 'function') {
           curr[name][exdate[name].toISOString().slice(0, 10)] = exdate[name];
         } else {
-          throw new TypeError('No toISOString function in exdate[name]', exdate[name]);
+          throw new TypeError('No toISOString function in exdate[name] = ' + exdate[name]);
         }
       }
     }
@@ -548,7 +548,7 @@ module.exports = {
             if (typeof curr.recurrenceid.toISOString === 'function') {
               par[curr.uid].recurrences[curr.recurrenceid.toISOString().slice(0, 10)] = recurrenceObject;
             } else { // Removed issue 56
-              throw new TypeError('No toISOString function in curr.recurrenceid', curr.recurrenceid);
+              throw new TypeError('No toISOString function in curr.recurrenceid =' + curr.recurrenceid);
             }
           }
 
@@ -623,10 +623,10 @@ module.exports = {
 
               rule = rule.replace(/\.\d{3}/, '');
             } catch (error) { // This should not happen, issue #56
-              throw new Error('ERROR when trying to convert to ISOString', error);
+              throw new Error('ERROR when trying to convert to ISOString ' + error);
             }
           } else {
-            throw new Error('No toISOString function in curr.start', curr.start);
+            throw new Error('No toISOString function in curr.start ' + curr.start);
           }
         }
 
@@ -645,11 +645,23 @@ module.exports = {
     URL: storeParameter('url'),
     UID: storeParameter('uid'),
     LOCATION: storeParameter('location'),
-    DTSTART(value, parameters, curr, stack) {
-      curr = dateParameter('start')(value, parameters, curr, stack);
-      return typeParameter('datetype')(value, parameters, curr);
+    DTSTART(value, parameters, curr, stack, line) {
+      // If already defined, this is a duplicate for this event
+      if (curr.start === undefined) {
+        curr = dateParameter('start')(value, parameters, curr, stack);
+        return typeParameter('datetype')(value, parameters, curr);
+      }
+
+      throw new Error('duplicate DTSTART encountered, line=' + line);
+    },
+    DTEND(value, parameters, curr, stack, line) {
+      // If already defined, this is a duplicate for this event
+      if (curr.end === undefined) {
+        return dateParameter('end')(value, parameters, curr, stack);
+      }
+
+      throw new Error('duplicate DTEND encountered, line=' + line);
     },
-    DTEND: dateParameter('end'),
     EXDATE: exdateParameter('exdate'),
     ' CLASS': storeParameter('class'), // Should there be a space in this property?
     TRANSP: storeParameter('transparency'),