Singular case:
رَجُلٌ ذُو عِلْمٍ — nominative.
رَأَيْتُ رَجُلًا ذَا عِلْمٍ — accusative.
مَرَرْتُ بِرَجُلٍ ذِي عِلْمٍ — genitive.
Dual contrast:
رَجُلَانِ ذَوَا عِلْمٍ — nominative.
رَأَيْتُ رَجُلَيْنِ ذَوَيْ عِلْمٍ — accusative.
An ordinary dual or sound masculine plural normally carries nūn, as in رَجُلَانِ and مُسْلِمُونَ. In these mudaf forms that nūn is absent: ذَوَا/ذَوَيْ، ذَوُو/ذَوِي، أُولُو/أُولِي. Also compare demonstrative هَذَا رَجُلٌ (“this is a man”) with رَأَيْتُ رَجُلًا ذَا عِلْمٍ; in the second phrase ذَا means “possessing,” not “this.”