How to Duplicate and Copy Slides in Google Slides
Once you create and modify some slides to make them just right, don't reinvent the wheel! Create copies to base your new slides on.
Press Ctrl + D or right-click the slide and select Duplicate Slide.
The slide is duplicated, and inserted right after the initially selected slide.
If you want to insert a duplicate of a slide in another part of the presentation, you can copy and paste entire slides as well
A copy of the selected slide is inserted at the selected spot in the presentation.