Having the AMG logo on the ducktail (or any other body part for that matter) has no correlation to authenticity. AMG made almost all their parts in homolgated and non-homolgated versions. The key to authenticity is having the AMG part number stamped on the part. Note, that some ducktails for the w124 did not have 'AMG' engraved but came with a stick on '///AMG' badge. You can still get these from MB dealership, set costs circa 400 gbp.
The AMG part number on the ducktail is stamped on the inner body of each of the 3 pieces and starts with 'C788....' on the earlier versions. Late version are all HWA 124 part numbered. I don't believe a ducktail was made for the facelift w124 though, at least i haven't seen one that works on the facelift bootlid.
Getting these off a car without damage to the original bodywork will be nigh impossible if the item was fitted properly in the first place.
talbir