The line comes from Homestar Runner, the long-running Flash cartoon series created by Mike and Matt Chapman, where Strong Bad is the boastful masked character known for Strong Bad Email. Clips from the series circulated widely on early meme and soundboard sites because the voices are distinct and the lines are short. This particular quote survived as a standalone reaction because it needs no setup, anyone hearing the surprise in his voice understands a duplicate has appeared, even without knowing the original cartoon scene or episode context at all.