Can serial numbers be duplicated

[geofflit]

Have discovered that a care plan warranty was in place for my laptop between 2011 and 2014. However I purchased the laptop in 2015 as new from a national store in UK. What are the chances of serial numbers for two laptops to be the same, especially years apart? 

[Huffer]

Zero. What model laptop is this? Likely did not exist in 2011. 

[geofflit]

Showing as HP Notebook with Product number  P0U71EA#ABU.

What would explain the expired warranty showing for the laptop?

Have concerns as company was fined in 2002 for selling old pcs as new.

Thanks

[Huffer · Accepted solution]

15-ac152sa is the model number. Intel 4th gen processor was not introduced until late 2013 at the earliest. So it could not have had a warranty in 2011, but I also do not believe it is a new laptop.