Same problem here with 27 mystery named ranges. I used Ctrl+F3 (as recommended in another answer) to view the list of named ranges (which I had never set up!). There were many since I had been copying the worksheet tab many times and just clicking "yes" to work around the message. I highlighted and deleted all the named ranges (one little screen at a time, couldn't Select All). The next time I tried to copy the worksheet, the copy succeeded without the warning message.
Be sure to copy your original file before trying this.