Identify duplicates and format all except the first one

Hello!

I am trying to identify the duplicates in a list of appointments for the same day.

In this list, one user can have multiple appointments. I would like to fomat  the rows for all of which are second, third or fourth appointments (leaving the first appointment for each user  non formatted).

I would like to then be able to  add the time  of duration of all the appointment  for each user  and add a column  to the first appointment showing me the end of the last appointment.

Thank you for your precious help!