Thanks @jim75924 Thanks again for clarifying that.
Yes you are right to say “…” [NOT] ‘right click’.
Summary
Duplicate is NOT showing for me from the same action.
(e.g. expected - see “Duplicate” on clicking “…” there dots)
I too was convinced that I normally see " Duplicate page " as in your screenshot FOR a dynamic page. I’ve reloaded page/can replicate same problem missing “Duplicate Menu” on Dynamic page…
Everything is working as expected for duplicating a static page
Perhaps I should report this issue to support team
?and change the title of this post? 
‘Duplicate Page’ - menu item NOT showing for Dynamic Page
“Mac OS: 11.3.1 Safari Version 14.1 (16611.1.21.161.6)”
Thanks again @jim75924 for confirming.