When you have System Action in the Finish step of the process it may cause same process to run more then once.
Steps to Reproduce:
Expected Result: Process only run once or at the worse twice as there is an action that updates the object and starting condition is simply on the Update Event.
Actual Result: Many process instances are triggered.
Applies to all supported PAS environments for specified releases.
Caused by CLRT-78999
This issue is currently in review with development.
Move any System Actions from the Finish step or adjust the process start conditions so that the process would not have a 'true' condition for starting another process instance prematurely.
Please note that whilst this discusses the Finish step specifically, and will be where the majority of problems originate from, any time the process finishes quickly after the last action completes could still cause the same problem, albeit less frequently than with the Finish step.