Wordpress URL Modification Querry

I was trying to model some sites who use the category in their url for a client. This is something I have never played with because I have always just %postname% as a custom and moved on.
SO I went and did this /%category%/%postname% and what I get is 2 presentations of the category in the url. ???? Why? I dunno.
That's why I thought I'd come in and ask what yáll thought and see what comes up...

Thanks for the reply in advance!
"Observation is an act of creation through limitations inherent in thinking"