The most likely cause for this problem is that the original feature was displayed using Pen Mode. In Pen mode displayed graphics are permanent which will allow for duplicates. Please go to Tools > Locks and ensure that your Pen/Pencil Lock is set to Pencil (the pencil is yellow, and pen is blue). Delete all features from graphics and redisplay them.
If this does not resolve your issue, please post any followup questions as a reply to this thread.