ERP5Workflow: make sure not create duplicate permissions

Fix a problem introduced in ff624fd2 (ERP5Workflow: newly added
permission should be acquired for all existing states., 2024-11-04),
visible in a test failure
6 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

01:16:15

failed ERP5.PerformanceTest-Master

00:35:32

failed ERP5.UnitTest-Master

03:45:32

passed SlapOS.Eggs.UnitTest-Master.Python2

00:24:59

passed SlapOS.Eggs.UnitTest-Master.Python3

00:24:28

passed Wendelin.UnitTest-Master

00:40:25