If you already have the previous episodes Sonarr should see that in your folder. If you don't have them, but have already seen them, you can manually select which ones not to download. If you go into the series, there is a little flag icon next to each season and episode. Clicking on that toggles that season or episode for Sonarr to look for it or not. A solid black flag means Sonarr will look for it, an outlined flag only means Sonarr will not look for it. Bit of a hassle to click on all the episodes, but you only have to do it once.