My requirement is to tag the different sources of leads coming from web portals. If duplicate lead comes from different portal, it should be tagged so that business from different portals can be tracked. Please note that lead is entering into the system via web services. i.e, Integration
Attach the Lead to Campaigns, and you'll be able to track all of the places a lead came from. This is the ideal way to track multiple lead sources.
You can create two fields at Lead object, External Lead Id and Source System. When you are creating leads capture those information.