I have a Sphere with this BP:
This Sphere move to a cube (AI2). In single player it works. But if I add a player, then a cube is more spawn. Why?
This has to do with Network Replication. Essentially the server spawns a cube which is replicated, and the client spawns a cube which is not. So the client sees two cubes while the server sees one. The correct way to resolve this is to put authority checks using a Switch has Authority branch in your blueprint which would ensure it would only spawn on the server (which would again get replicated to the client).