Is it orginal or a replica?

Hi all, sometimes you can find on ebay some great design objects at very interesting prices. In the description the seller write the designer of the object but what usually is not known is the manufacturer of the object. So, I' think that an object in order to be original must have the original designer (obviously) and must be done by a licensed manufacturer...but for some objects it's not so easy (and not only on furniture shop too) to identify the manufacturer and if it is authorized. Sometimes these objects are very well done....this is the problem!! For example the Barcelona Armchair by Mies Van Der Rohe or the Bauhaus Chaise Longue. You can find dozen of these, but how to identify the originals? For every design object there is only one authorized manufaturer?
Thanks for your help!! Nic
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