How do I extract the user specific details in a form. Say if two different users are actioning on the same form, can their activities be differentiated/validated by some variable like userid?

I am developing a form in PDF Designer where multiple users action the same form. Based on the action of one user, the other person will action the next steps.

But a chance of same person actioning the form both the times exists. How can I answer this?

Is there a way where I can extract the userid of the person actioning the first time and use it to check who is actioning it the second time and prevent the same user from actioning twice!

Please help....urgent!