What if duplicate wallet addresses are created? I'm a newbie to Bitcoin, so I apologize if this seems like a "noob" question.

So, from what I've read: anyone can create a new Bitcoin wallet. Upon creation, a wallet address is automatically generated. The wallet address can be used by others to make transactions to that particular wallet.

Now, my question is, what guarantees that two automatically generated wallet addresses will be unique? If my friend and I created separate wallets on our own computers offline, then what is the guarantee that the two wallet addresses will be unique? What if the two randomly generated wallet addresses turned out to be the same?
The probability is unfathomably small. In practice it will never happen. You can find one billion friends all keep generating trillions of addresses every second and there won't be any duplicate addresses til the end of the life of the sun.

Edit: this actually exaggerated a bit. By virtue of the Birthday Paradox you only need to generate 2^80 addresses (not 2^160) to likely get a pair of duplicate addresses among all the addresses generated. So this address generating party won't be able to run that long.

On the other hand if your billion friends all keep generating addresses in an attempt to get the same address as yours, then it does take forever to happen.