A particular case of dipole is the so-called doublet, in which the quantity a tends to zero so that the source and sink both move towards the origin.
The complex potential of a doublet
is obtained making the limit of the dipole potential for vanishing a with the constraint that the intensity of the source and the sink must correspondingly tend to infinity as a approaches zero, the quantity
being constant (if we just superimpose a source and sink at the origin the resulting potential would be W=0)
Hint: Develop and in a Taylor series in the neighborhood of the
origin, assuming small a