Final answer:
Claudius' backup plan for the duel in 'Hamlet' is to poison a goblet of wine for Hamlet to drink if Laertes' blade fails to kill him.
Step-by-step explanation:
Claudius' backup plan for the duel in Hamlet involves a poisoned goblet of wine. If Laertes' poisoned sword fails to kill Hamlet, Claudius intends for Hamlet to drink from the poisoned cup during a break in the duel. The wine is tainted with a potent poison, ensuring that if Hamlet were to drink it, he would not survive. This plan is a reflection of Claudius' deceitful and manipulative character, as he schemes to ensure Hamlet's death while maintaining a facade of goodwill towards him.