Resonant in situ photoemission from Mn 3d states in Ga_1−xMn_xAs is reported for Mn concentrations down to the very dilute level of 0.1 per cent. Concentration-dependent spectral features are analyzed on the basis of first principles calculations for systems with selected impurity positions as well as for random alloys. Effects of direct Mn-Mn interaction are found for concentrations as low as 2.5 per cent, and are ascribed to statistical (nonuniform) distribution of Mn atoms.