Doppelgänger pandemic

(Forgive me, I’m not technical or smart enough to properly make a scenario.) It’s a silent apocalypse outside. No one openly says it, but people get switched out with something that’s acting like them, almost perfectly, but they’re… off. You don’t know what happened to some of your neighbors, but when they got switched, nobody knows what happens to the original. Riots broke out, but it didn’t help because some of the rioters go missing and replaced a few hours later. It became normal to start boarding up your windows and barricading the doors. However, how long can you last when the thought of you or someone close to you being replaced by something else?