A multi-particle finite-element method was proposed to study the elastic-plastic behaviour of ductile powders composed of highly deformable elastic-plastic particles. The focus was put on the study of the uniqueness of the direction of plastic strain increment vectors for a given stress state on the plastic limit, which was assessed using a spherical stress-probing method. Results revealed a non-uniqueness of the direction of plastic flow in a small region of the stress space located in the vicinity of the loading point. The direction of plastic flow was almost unique elsewhere on the plastic limit. The non-uniqueness was explained using a combination of two distinct mechanisms for plastic deformation involving two very different plastic limits.