I assume you used a 35mm film camera to do actual double exposure. If so, They will look like ghosts if they overlap anything that is bright enough to show through your subject. If your subject is wearing bright clothing, like white, for example, dark areas behind them should not show through. It all has to do with brightness, darkness, and placement of the model with other elements.
Maybe you could try combining images in post editing. You will have more control of placing them.