P60 Certificate of Pay, Income Tax and National Insurance Contributions
In the United Kingdom a P60 is issued to you by your employer at the end of the financial year. This document is essentially a sheet of paper, printed with fixed-width font, detailing your earnings, tax deductions, and National Insurance contributions for the year.
There is a warning at the top, telling you not to destroy it. At the bottom it tells you that a duplicate cannot be issued. I don't quite see the point in this, because I wouldn't think one could tell an original from a photocopy. Not to mention that I could easily produce an identical P60 in Notepad (or any text editor) featuring arbitary figures.
It sounds like they are placing trust in you only having one P60, for whatever reason, by restricting the issue of duplicates. Unless it is printed on special paper, or printed with magnetic ink, or uses some other clever methods, I can't see why it should be distinguishable from a photocopy, or indeed something produced on a home computer.
The P60 comes into play when you are dealing with Inland Revenue. For example, when going through the complicated procedure to claim back extra tax they deducted from your income as a precautionary measure and weren't planning to give back until you hassle them.