When you want to trigger different actions on a redispatch vs. a dispatch, you can use following triggers and actions:
For a first dispatch
Trigger: status changed to checked-in
Condition: current / previous status is not checked-in
For a redispatch:
Trigger: status changed to checked-in
Condition: current / previous status is checked-in
Note that a redispatch is the only situation where a status change from checked-in to checked-in is possible.