It is said that each individual Pokemon have (are born with) their own individual values (commonly abbreviated as IV's). Having said that, it seems the greatest possible difference between two similar pokemons with the same CP is only around 10%. So by the time you fully max out two of the same Pokemon, one will most likely have higher stats than the other (attack, defence, stamina)

So if one is looking to get THE perfect Pokemon, those are the stats they will be looking at - even if it is only 10% max difference.

There are calculators online to help determine the figures and whether or not you have an average version of your particular Pokemon or whether you have an "elite" version.

It can also help you determine which of the duplicates you might want to invest in stardust on over the other when the time comes to want to power them up.