When we migrate workflow, we also migrate workflow step securities. A migrated workflow replaces an existing workflow of the same name, therefore in likewise manner, workflow step security should replace the same workflow step security. However, workflow step security has no name, so we need to match them by the properties that makes them unique, which is a combination of security type and security group/user/token and use the same ID. Obviously, if you have two entries that has the same properties, then they will map to the same ID. Hence the error.
What you CAN do is try to import the workflow, after changing the workflow name - this should work. If that's not an option and assuming that you're on a supported PPM-version I would log a case. Then please also supply the following information:
Select max(WORKFLOW_STEP_SECURITY_ID) from KWFL_WORKFLOW_STEP_SECURITY;
select KWFL_WORKFLOW_STEP_SECURITY_S.nextval from dual;
select * from KWFL_WORKFLOW_STEP_SECURITY;
select KWFL_WORKFLOWS_S.NEXTVAL from dual;
SELECT WORKFLOW_STEP_ID, SECURITY_GROUP_ID, count(*) FROM KWFL_WORKFLOW_STEP_SECURITY where SECURITY_GROUP_ID IS NOT NULL GROUP BY WORKFLOW_STEP_ID, SECURITY_GROUP_ID HAVING count(*) > 1;