2.2.152 [RFC5546] Section 5.3 Sequence Number

V0318:

The specification states that under some conditions, a calendar user agent may receive requests and replies with the same SEQUENCE property value. The DTSTAMP property is utilized as a tie-breaker when two items with the same SEQUENCE property value are evaluated.

Microsoft Office Outlook 2007, Microsoft Outlook 2010, Microsoft Outlook 2013, Microsoft Outlook 2016, Microsoft Outlook 2019

Outlook allows the user to respond multiple times to a single meeting request. In these cases, Outlook does export items with the same UID and SEQUENCE number. Outlook increments the sequence number every time the organizer sends out an update to a meeting, but not every time the organizer saves a meeting to an .ics file.

On import, Outlook uses the DTSTAMP property as a tie-breaker when evaluating items with the same UID and SEQUENCE property values.