Thanks all! With your help, I managed to get what I want - thusly.
Settings->File Types ..->All Files->Context Menu->New.- Change the 'New Action' text to 'Duplicate'.
- Put
Copy DUPLICATE HERE PATTERN * AS * WHENEXISTS=renameinto the text box. - Press 'OK'. (One does not need to close the 'File Types') menu.
I note that at least one file manager (namely Linux's 'Nemo') has a 'duplicate' option by default. (Or rather, if I remember correctly, Nemo has a range of context-menu items that one can enable/disable via tickboxes.)