Duplicated content because of one /

Hi guys,

I've got a website where I have no idea why but there are a lot of sites like this

website.com/category/site01
website.com/category/site01/

Of course both are loaded into the browser and of course they are one and the same. Yeah, as a result we've got duplicated content. I plan to buy this site and I am wandering what could be the reason for this shit. I am pretty sure that this is not some kind of mumbo-jumbo because I know the seller (he has no any explanation). I'm almost sure that this is some mistake in the coding. The site is not in english and i can't post it here

I'm asking just for a hint

Thanks