A doublet can be thought of as a combination of a source and a sink of equal strengths kept at an infinitesimally small distance apart. Thus the streamlines can be seen to start and end at the same point.
The strength of a doublet made by a source and sink of strength  kept a distance  is given by –
The velocity of fluid flow can be expressed as –
The equations and the plot are for the limiting condition of 
The concept of a doublet is very similar to that of electric dipoles and magnetic dipoles in electrodynamics.