Be able to duplicate a page with the right button of the mouse
Thank you for valuable feedback. I’m glad to inform that we have now enabled the functionality to duplicate pages in Visio for the web. You can duplicate a particular page via the right-click menu. Please refer to our help article to learn more https://support.microsoft.com/en-us/office/copy-pages-and-shapes-in-a-visio-drawing-bf5c5262-0688-4306-a1ac-748e406b0e4f?ui=en-US&rs=en-001&ad=US#OfficeVersion=Web or our recent blog post about it https://techcommunity.microsoft.com/t5/visio-blog/we-heard-you-diagramming-is-even-easier-in-visio-for-the-web/ba-p/1670427
To read more about the new features and functionalities please refer to our recent blogs at https://techcommunity.microsoft.com/t5/visio-blog/bg-p/VisioBlog
It's already available in the last version (I don't know since which version).