Yes, it can be tremendously difficult and I don’t even need to address the issue of how “copy and paste” isn’t really a thing in programming.
The thing is this:
Your argument is that this was done over 10 years ago so…
By whom?
How has the code changed in between that and now?
How is the documentation?
Did the original code have any flaws that would need to be ironed out or in a different way, make the need to redesign it completely?
These aren’t even the biggest questions one can ask but they are among the most simplest to describe in laymen term. So no, the idea of “You did this 10+ years ago” doesn’t work in regards to coding… it barely works in any regard due to ya’ know… 10+ years have passed making A LOT OF STUFF very different from when they were first done; sometimes for the better, sometimes for the worse but always in a different way.