Hover over the copy button (double box icon, left slide menu)
Select copy
Duplicate: instantly makes a slide next to your original
Copy: saves the slide to your clipboard
Click on the slide you want to paste after (in the left thumbnail menu of any deck), then right-click and select "paste"
Keyboard shortcuts
The fastest route 🚀
On Windows:
Select slide → Copy: Ctrl + C → go to deck you want to paste into and select the slide you want to paste after → paste: Ctrl + V
On Mac:
Select slide → Copy: Command + C → go to deck you want to paste into and select the slide you want to paste after → paste: Command + V