Only do duplicate parameter check for PI-XML

Before, we would (optionally) check if a parameter read from the PI-XML files (rtcParameterConfig*.xml) was already set before. This would lead to a warning if a parameter occurred in multiple PI-XML files but also for example if it was already present in the model file(s). The latter type of warning is unnecessary as we expect parameters to be updated at runtime. This commit makes sure that only warnings are raised for duplicates found in the PI-XML files.