You can copy a slide from one deck to another by pressing the insert slide button, followed by clicking the "My Decks" tab. Under "My Decks" you can browse and insert slides from any of your other decks.
Slide TemplatesIf you regularly need to insert the same slide, you can save save it to your slide templates library for more convenient access. Start by navigating to the slide you want to save, press the insert slide button to bring up the slide templates and finally click "Save current slide" at the bottom.
More about slide templates here: http://help.slides.com/knowledgebase/articles/405645-slide-templates