Proper motion in lensed radio jets at redshift 3: a possible dual
  super-massive black hole system in the early Universe
  In this paper, we exploit the gravitational lensing effect to detect proper
motion in the highly magnified gravitationally lensed source MG B2016+112. We
find positional shifts up to 6 mas in the lensed images by comparing two Very
Long Baseline Interferometric (VLBI) radio observations at 1.7 GHz that are
separated by 14.359 years, and provide an astrometric accuracy of the order of
tens of $\mu$as. From lens modelling, we exclude a shift in the lensing galaxy
as the cause of the positional change of the lensed images, and we assign it to
the background source. The source consists of four sub-components separated by
$\sim 175$ pc, with proper motion of the order of tens $\mu$as yr$^{-1}$ for
the two components at highest magnification ($\mu\sim350$) and of the order of
a few mas yr$^{-1}$ for the two components at lower magnification ($\mu\sim2$).
We propose single AGN and dual AGN scenarios to explain the source plane.
Although, the latter interpretation is supported by the archival
multi-wavelength properties of the object. In this case, MG B2016+112 would
represent the highest redshift dual radio-loud AGN system discovered thus far,
and would support the merger interpretation for such systems. Also, given the
low probability ($\sim10^{-5}$) of detecting a dual AGN system that is also
gravitationally lensed, if confirmed, this would suggest that such dual AGN
systems must be more abundant in the early Universe than currently thought.
