This paper compares, for the first time, laser powder bed fusion (PBF-LB/M) processing of a powder mixture (PM), also known as in-situ alloying, with that of a pre-alloyed (PA) powder from gas atomization with the same chemical composition, using the example of X2CrNiMoN20–10–3 ferritic-austenitic stainless steel. The focus is on the differences in the microstructure formation mechanisms during PBF-LB/M between PM and PA using different energy inputs, in order to gain new insights into the process transferability of in-situ alloying to the processing of pre-alloyed powders. The microstructure investigations are carried out using electron backscattered diffraction (EBSD), energy dispersive (EDS) as well as wavelength dispersive X-ray spectrometry (WDS), X-ray diffraction (XRD) and magneto-inductive method (Feritscope®). The microstructures of samples produced from PM and PA differ significantly in terms of the resulting ferritic and austenitic phase fractions, so that a ferritic-austenitic microstructure forms for PM, while the PA is predominantly austenitic. The differences are mainly based on the increased chemical inhomogeneities for the PM in comparison to the PA state, which are discussed based on EDS map analysis through spatial statistics. With increasing energy input, the chemical homogeneity of the PM approaches that of the PA, but it cannot reach it even with maximum energy input. The formation of a ferritic-austenitic microstructure in the case of the PM leads to the formation of a finer microstructure compared to single-phase PA steel resulting in higher hardness of PBF-LB/M-built PM.