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.
About this sound
Strong Bad says there's two of them in this short Homestar Runner clip, used as a reaction sound for surprising doubles, copies or unexpected pairs.
This is Strong Bad's deadpan voice line from Homestar Runner, delivered as he notices there are suddenly two of something. His gruff, masked wrestler voice stretches the disbelief just enough to make a simple count sound funny. The clip is short and clean, with the emphasis landing on 'two' before the line ends abruptly. That flat, mildly alarmed delivery is why it works so well out of context, turning an ordinary duplicate, lookalike or second enemy appearing on screen into a small comic moment of realization and mild panic for the viewer watching.