Given the attached SW the following behavior was detected.

1) we create an instance by passing NO workflow data, and the following header parameter Authorization MY_TOKEN

The result below is returned. Which is correct, since the SW is grabbing the header and storing in the "message" field.

{
    "id": "cd87803e-0a9e-4536-a9df-5fd57b28c1a3",
    "workflowdata": {
        "message": " The Authorization header is MY_TOKEN"
    }
}

2) we create a second instance by passing NO workflow data and we pass the same header and value.

This time we get the following result:

 
{
    "id": "9fb2b5db-999e-485b-831b-161cdc555b51",
    "workflowdata": {
        "message": " The Authorization header is MY_TOKEN The Authorization header is MY_TOKEN"
    }
}

note that the message is duplicated.

3) if we execute another time repeating the header, etc, we get this result.

{
    "id": "9c8b3160-8988-4d79-9783-9d792316def1",
    "workflowdata": {
        "message": " The Authorization header is MY_TOKEN The Authorization header is MY_TOKEN The Authorization header is MY_TOKEN"
    }
}
 

NOTE:

If we pass a workflowdata, then it works as expected, i.e, no duplication.

So it looks like it's something related with the by default workflow data that is created when value is passed to the SW.