Using the lens formula,
\(\frac{1}{v} + \frac{1}{u} = \frac{1}{f}\)
Where u, v and f are the object distance, image distance and focal length respectively.
\(\therefore \frac{1}{2f} + \frac{1}{u} = \frac{1}{f}\)
\(\frac{1}{u} = \frac{1}{2f} - \frac{1}{f} \implies \frac{1}{u} = \frac{1}{2f}\)
\(u = 2f\)
\(\therefore Magnification = \frac{\text{image distance}}{\text{object distance}}\)
= \(\frac{2f}{2f}\)
= 1