diff --git a/NEWS b/NEWS
index 190c1da46..beb3ba617 100644
--- a/NEWS
+++ b/NEWS
@@ -14,6 +14,7 @@ Bug fixes
  - EAS fix on qp-encoded subjects (#3390)
  - correctly handle all-day event exceptions when the master event changes
  - prevent characters in calendar component UID causing issues during import process
+ - avoid duplicating attendees when accepting event using a different identity over CalDAV
 
 2.3.3a (2015-11-18)
 -------------------
diff --git a/SoObjects/Appointments/SOGoAppointmentObject.m b/SoObjects/Appointments/SOGoAppointmentObject.m
index 4bf315e71..a1a840ebe 100644
--- a/SoObjects/Appointments/SOGoAppointmentObject.m
+++ b/SoObjects/Appointments/SOGoAppointmentObject.m
@@ -2173,13 +2173,37 @@ inRecurrenceExceptionsForEvent: (iCalEvent *) theEvent
             {
               iCalPerson *attendee, *delegate;
               NSString *delegateEmail;
-              
-              attendee = [newEvent userAsAttendee: [SOGoUser userWithLogin: owner]];
+
+              attendee = [oldEvent userAsAttendee: [SOGoUser userWithLogin: owner]];
+
+              if (!attendee)
+                [newEvent userAsAttendee: [SOGoUser userWithLogin: owner]];
+              else
+                {
+                  // We must do an extra check here since Bob could have invited Alice
+                  // using alice@example.com but she would have accepted with ATTENDEE set
+                  // to sexy@example.com. That would duplicate the ATTENDEE and set the
+                  // participation status to ACCEPTED for sexy@example.com but leave it
+                  // to NEEDS-ACTION to alice@example. This can happen in Mozilla Thunderbird/Lightning
+                  // when a user with multiple identities accepts an event invitation to one
+                  // of its identity (which is different than the email address associated with
+                  // the mail account) prior doing a calendar refresh.
+                  NSMutableArray *attendees;
+                  NSString *partStat;
+
+                  attendees = [NSMutableArray arrayWithArray: [newEvent attendeesWithoutUser: [SOGoUser userWithLogin: owner]]];
+
+                  partStat = [newEvent participationStatusForUser: [SOGoUser userWithLogin: owner]
+                                                         attendee: attendee];
+                  [attendee setPartStat: partStat];
+                  [attendees addObject: attendee];
+                  [newEvent setAttendees: attendees];
+                }
               
               // We first check of the sequences are alright. We don't accept attendees
               // accepting "old" invitations. If that's the case, we return a 403
               if ([[newEvent sequence] intValue] < [[oldEvent sequence] intValue])
-                return [NSException exceptionWithHTTPStatus:403
+                return [NSException exceptionWithHTTPStatus: 403
                                                      reason: @"sequences don't match"];
               
               // Remove the RSVP attribute, as an action from the attendee
diff --git a/SoObjects/Appointments/iCalEntityObject+SOGo.h b/SoObjects/Appointments/iCalEntityObject+SOGo.h
index 88e7d3b24..1f0365cf2 100644
--- a/SoObjects/Appointments/iCalEntityObject+SOGo.h
+++ b/SoObjects/Appointments/iCalEntityObject+SOGo.h
@@ -44,6 +44,9 @@ extern NSNumber *iCalDistantFutureNumber;
 
 - (iCalPerson *) userAsAttendee: (SOGoUser *) user;
 
+- (NSString *) participationStatusForUser:  (SOGoUser *) theUser
+                                 attendee: (iCalPerson *) theAttendee;
+- (NSArray *) attendeeUIDs;
 - (BOOL) isStillRelevant;
 
 - (id) itipEntryWithMethod: (NSString *) method;
diff --git a/SoObjects/Appointments/iCalEntityObject+SOGo.m b/SoObjects/Appointments/iCalEntityObject+SOGo.m
index 2fd9f9cae..9d859c6e5 100644
--- a/SoObjects/Appointments/iCalEntityObject+SOGo.m
+++ b/SoObjects/Appointments/iCalEntityObject+SOGo.m
@@ -422,8 +422,8 @@ NSNumber *iCalDistantFutureNumber = nil;
 
 - (iCalPerson *) userAsAttendee: (SOGoUser *) user
 {
-  NSEnumerator *attendees;
   iCalPerson *currentAttendee, *userAttendee;
+  NSEnumerator *attendees;
 
   userAttendee = nil;
 
@@ -436,6 +436,34 @@ NSNumber *iCalDistantFutureNumber = nil;
   return userAttendee;
 }
 
+- (NSString *) participationStatusForUser:  (SOGoUser *) theUser
+                                 attendee: (iCalPerson *) theAttendee
+{
+  iCalPerson *currentAttendee;
+  NSEnumerator *attendees;
+  NSMutableArray *a;
+
+  a = [NSMutableArray array];
+
+  attendees = [[self attendees] objectEnumerator];
+  while ((currentAttendee = [attendees nextObject]))
+    if ([theUser hasEmail: [currentAttendee rfc822Email]])
+      {
+        // If the attendee is the same but the partStat is
+        // different, we prioritize this one.
+        if ([[currentAttendee rfc822Email] caseInsensitiveCompare: [theAttendee rfc822Email]] == NSOrderedSame &&
+            [[currentAttendee partStat] caseInsensitiveCompare: [theAttendee partStat]] != NSOrderedSame)
+          [a insertObject: currentAttendee  atIndex: 0];
+        else if ([[currentAttendee rfc822Email] caseInsensitiveCompare: [theAttendee rfc822Email]] != NSOrderedSame)
+          [a addObject: currentAttendee];
+      }
+
+  if ([a count] > 0)
+    return [[a objectAtIndex: 0] partStat];
+
+  return [theAttendee partStat];
+}
+
 - (BOOL) userIsOrganizer: (SOGoUser *) user
 {
   NSString *mail;