The properties of duplex stainless steels depend strongly on their thermal history, which can produce a wide range of austenite to ferrite ratios; whereas optimal properties generally require near 50-50 ferrite-austenite duplex microstructures. Additive manufacturing processes of duplex steels remain challenging as it is difficult to predict and control how the phase ratio depends on process parameters. This paper focuses on directed energy deposition additive manufacturing and presents a fast numerical modelling of the thermal history and diffusion controlled solid-solid phase transformations in the entire part. The proposed simulations strategy is sufficiently fast to optimize the process parameters to achieve a targeted distribution of phase ratio, and a temperature control strategy of the build platform has been proposed on this basis to reach almost uniform near 50-50 phase ratios, which was obtained by setting the temperature profile of the build platform as a linear function decreasing from 1000~K for the first layer to 800~K for the last layer. In addition, experiments are conducted to validate the proposed approach. Microstructures and phase ratio gradients are assessed in single-bead-thickness walls of SAF~2507 superduplex stainless steel, and numerical results are in reasonable agreement with experimental observations.