If it is already open and you don't have its handle, then first get it
fig = gcf(); % handle of current figure;
pos = fig.Position; % contains its position and dimensions
pos is 4 element vector [x y w h]. x and y are the positions of the bottom left corner of the figure window in screen coordinates (by default in pixels). w and h are the width and height of the window (by default in pixels too). Create a new figure at the same position and with the same dimensions
fig2 = figure('Position', pos)
