Parallel Flow Launcher trigger workflow twice even when the max flow is only 1
Workflow activities triggered twice
This usually happens when the workflow run parallel flow launcher activity twice. You can check this by going to the Workflow Context of the record > Go to the 'Workflow Activity History' tab > Filter the Activity (name) of the parallel flow launcher and see it logged twice.
The issue can also be applied on other types of activities
Check the Workflow Activity History and check which activities ran twice.
Trace it back to the first activity and see if it made two workflow path or transition to the parallel flow launcher activity. The below workflow has a design that can trigger an activity twice. In this sample, Parallel Flow Launcher was triggered twice.
Workflow Path 1: (in red) If activity > Catalog Task 1 > Catalog Task 3 > Parallel Flow Launcher
Workflow Path 2: (in black) If activity > Catalog Task 2 > Wait for Condition > Catalog Task 1 > Catalog Task 3 > Parallel Flow Launcher
You will need to redesign your workflow in order to prevent triggering the same activity twice.