They don't. These addresses are generated randomly and chances are so damn small that this never happens. Bitcoin is decentralized, so these is no "central point" that shows which address to generate or which ones were already used.
People already discussed this multiple times in the past.
What happens if 2 people generate the same bitcoin wallet address?This would be called a "collision" and is highly unlikely.
Keys are 256 bit in length and are hashed in a 160 bit address.(2^160th power) Divide it by the world population and you have about 215,000,000,000,000,000,000,000,000,000,000,000,000 addresses per capita.(2.15 x 10^38)