On my Windows laptop, I'm using Outlook 2010 to fetch my Gmail account (IMAP). Gmail has its own spam folder + spam filter, and I configured Outlook to use the Gmail spam filter.
Nevertheless, Outlook always creates a second spam folder (in Dutch called: Ongewenste e-mail), which makes it complicated on other clients (or webclient) to know which one is the correct label.
I just learned you can hide labels in your webclient and/or IMAP clients by logging in into Gmail webaccount > Settings > Labels . I also marked some other labels to be hidden as well.