  We report on the surprisingly high metallicity measured in two absorption
systems at high redshift, detected in the Very Large Telescope spectrum of the
afterglow of the gamma-ray burst GRB 090323. The two systems, at redshift
z=3.5673 and z=3.5774 (separation Delta v ~ 660 km/s), are dominated by the
neutral gas in the interstellar medium of the parent galaxies. From the singly
ionized zinc and sulfur, we estimate oversolar metallicities of [Zn/H]
=+0.29+/-0.10 and [S/H] = +0.67+/- 0.34, in the blue and red absorber,
respectively. These are the highest metallicities ever measured in galaxies at
z>3. We propose that the two systems trace two galaxies in the process of
merging, whose star formation and metallicity are heightened by the
interaction. This enhanced star formation might also have triggered the birth
of the GRB progenitor. As typically seen in star-forming galaxies, the
fine-structure absorption SiII* is detected, both in G0 and G1. From the
rest-frame UV emission in the GRB location, we derive a relatively high, not
corrected for dust extinction, star-formation rate SFR ~ 6 Msun/yr. These
properties suggest a possible connection between some high-redshift GRB host
galaxies and high-z massive sub-millimeter galaxies, which are characterized by
disturbed morphologies and high metallicities. Our result provides additional
evidence that the dispersion in the chemical enrichment of the Universe at high
redshift is substantial, with the existence of very metal rich galaxies less
than two billion years after the Big Bang.
