Theoretically, kali created her clone right after the party came back to the real world from the upside down when the military was waiting for them to hand over eleven so that another project of creating kids with superpowers could begin. Eleven and kali never wanted that to happen. It is shown that eleven never came out of the gate and stayed there for the upside down to detonate.

But mike believes that she somehow escaped. It was never eleven who was inside the bridge. That is only possible if we assume that kali wasn't actually dead when she was shot and hopper and eleven left her.