The copy command is commonly represented by two document icons overlapping each other. In some variations an arrow points from one document to the other. The copy command is part of the cut-and-paste metaphor.
The cut, paste and copy commands were part of the Xerox text editors in 1974. The copy icon appears at least in the 2.0 version of Microsoft Word that was released in 1991.
Microsoft Word 2.0 1991.