Modifying a built-in R function

Hello,

Something incredible (at least for me) has happen. Yesterday night I
downloaded biplot.R to edit this function and add new features I wished.
Namely I wanted to plot points belonging to different groups using different
colors and symbols. I identified which part of the original code I had to
modify. Then, I rename biplot by biplotes and executing biplotes(x), being x
a princomp class object, the function did what I wanted.

The problem is that today I type exactly the same (after sourcing my script
file incluiding biplotes) but  biplotes(x) execute the original biplot
function. Also, if I invoke any of the new arguments I wrote in the code
then multiple warnings messages are displayed. I don't understand what is
the problem. Yesterday it works perfectly. Why R does not execute my code
and call the original biplot function?

Thanks in advance,
Javier.