Very close pairs of Quasi-Stellar Objects
  It is pointed out that there are now known four very close pairs of QSOs with separations < 5 arcsec and very different redshifts. Several estimates of the probability that they are accidental configurations range between 10^{-7} and 3.5 x 10^{-3}. We conclude either that this is further evidence that QSOs have significant non-cosmological redshift components, or that thave significant non-cosmological redshift components, or that the pairs must be explained by gravitational lensing. 
G. Burbidge, F. Hoyle and P. Schneider