Series 1 episode 5 when Chandler & Phoebe both break up with someone at the same time is the 1st time Janice is in the show, so we don't see when Chandler 1st met her.
The internet 'girlfriend' turns out to be Janice!!. Quite a while after splitting with Janice and not having much luck finding a new girl, he gets excited to have found what he thinks is perfect date through an internet chat room - which of course turns out to be his ex-girlfriend Janice - which is why everyone knows her.
i think after that episode they may have began seeing each other again - not sure?