Hi Bon thanks for posting in the Lucid community - I'm happy to help!

Yes it's possible to manage multiple slides at once! To do so hold Ctrl/Cmd which clicking to select each slide you want to edit. You can click either on the slide border directly on the canvas or on each slide in the Slides panel as demonstrated in this GIF:

A follow-up question to your note about the slides duplicating when you copy and paste the page: when you do so are you selecting all of the content on the page and then copying and pasting it onto a new tab or are you duplicating the tab itself at the bottom of the editor?

If you duplicate the page as shown above this also duplicates any other elements related to the page such as the presentation slides. I believe this is what you're experiencing! However if you use Ctrl/Cmd+A or click and drag to select everything on your page then copy and paste it onto a new blank page this will transfer only the shapes on the page and not duplicate your presentation slides. 

Give this a try and please let me know if I've misunderstood what you're experiencing or if you have any additional questions.