Manda,
No worries there - be sure to let them know that you are bringing them back with you and if they still charge you duty, simply keep your duty receipt. When you are leaving the island, show your receipt to the customs agent and ask for a reimbursement on the charges. Since you still have the items with you, they are not subject to duty and your money will be returned.
If you plan on leaving them on the island, then they are subject to duty as you could purchase most everything you are bringing on the island.
A single laptop and phone would never normally attract duty. But bringing in several laptops and several phones would probably attract a charge. The customs agents are skilled at their jobs and can usually determine if someone is trying to cheat the system...
