You can drag files while holding down CTRL to copy them.
Keyboard independent solution using a Nautilus script
Here's a short Nautilus script that should cover your use case:
#Nautilus script to duplicate selected files/folders
while [ $# -gt 0 ]; do
echo "$# file(s)/folder(s) left to duplicate."
cp -r "$ORIGINALITEM" "$DUPLICATEITEM"
Note: This is a very simple script and will fail if a duplicate file of the same naming scheme already exists in the same directory. It's also not as elegant as the naming Nautilus does by default ( (copy) is inserted after the extension).