If you are invited to an event but not the owner of the event, then you will see two events: (1) one for the event owner and (2) one for you, the attendee.

Why do I see two events for the same event in the matter calendar and matter gantt?

This is expected behavior. AdvoLogix's behavior of expanding and collapsing events depends on how Salesforce saves the events in the event object. If Salesforce identifies the parent/child events differently in these two situations, it will affect how they are displayed in the matter calendar as well. If Salesforce did not mark certain events as parent/child, it would affect our components.

Workaround

There is currently no workaround for this issue, as it is dependent upon Salesforce functionality.