add request header to all calls

I am trying to find a better solution for adding a header to all calls. This questions was already asked here and earlier here. The proposed solution for both of these was to add a RequestFilter.filterRequest event:

def headers = request.requestHeaders
headers.put( "X-test", "test" )
request.requestHeaders = headers

This solution has two problems:

1. This just keeps adding the same header for each call; meaning if you make the call say three times, you have three of these headers. I tried adding a RequestFilter.afterRequest event with request.requestHeaders.clear(), but that did not work.
2. I have multiple services in my project, and I need to add this header for all requests of only one of my services.

Any advice would be appreciated.