I think what you are wanting is domain name parking. if you have domain name with webpages called lets say, firstwebsite.com, you can purchase any number of domain names lets say website2.com,website3.com and asked your hosting company to park them ontop of firstwebiste.com.
people typing in website3.com or website2.com will automaticlly get the indexwebpage of firstwebsite.com, even better , depending on how you have you hyperlinks, all webpages will be disaplyed as if the parked domain was the original and no one need to know it is parked, unless you have some clues or your hyperlinks include the oringial site name etc.
Best to check your host to see if you are allowed to park domain names and how many you can have, some companies charge you a bit extra for this. All domain names must be paid for regardless of what you do with them .
hope this helps
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