How do I calculate the time spent between the same activities that occur multiple times in a process?

For the first and last activity, you can use the function:

CALC_THROUGHPUT

There is documentation about it on your help page, via your endpoint and: /help/display/CIBC/CALC_THROUGHPUT

If you like to do it separate for each, you can use the function use source target and time difference.

There is an article on your Celonis help page:

/help/display/CIBC/Throughput+Times

Example:

DAYS_BETWEEN ( SOURCE ( "Activities"."Timestamp" ) , TARGET ( "Activities"."Timestamp" ) )

Via using remap values, you can then get the activities you don't want to null e.g.

SOURCE ( "Activities"."Activity" , REMAP_VALUES ( "Activities"."Activity" , [ 'B' , null ] ) )

It is also possible with using ACTIVITY_LAG and ACTIVITY_LEAD and then calculating the time difference.

So many calculations possible :).

Hope it helps.

In Celonis Help - Search for Ping pong cases. It might answer your query.