  The rich open cluster M67 is known to have a chemical composition close to
solar, and an age around 4Gyr. It thus offers the opportunity to check our
understanding of the physics and the evolution of solar-type stars in a cluster
environment. We present the first spectroscopic study at high resolution,
R~50,000, of the potentially best solar twin, M67-1194, identified among
solar-like stars in M67. Based on a pre-selection of solar-twin candidates
performed at medium resolution by Pasquini et al. (2008), we explore the
chemical-abundance similarities and differences between M67-1194 and the Sun,
using VLT/FLAMES-UVES. Working with a solar twin in the framework of a
differential analysis, we minimize systematic model errors in the abundance
analysis compared to previous studies which utilized more evolved stars to
determine the metallicity of M67. We find M67-1194 to have stellar parameters
indistinguishable from the solar values, with the exception of the overall
metallicity which is slightly super-solar ([Fe/H]=0.023 +/- 0.015). An age
determination based on evolutionary tracks yields 4.2 +/- 1.6Gyr. Most
surprisingly, we find the chemical abundance pattern to closely resemble the
solar one, in contrast to most known solar twins in the solar neighbourhood. We
confirm the solar-twin nature of M67-1194, the first solar twin known to belong
to a stellar association. This fact allows us to put some constraints on the
physical reasons for the seemingly systematic departure of M67-1194 and the Sun
from most known solar twins regarding chemical composition. We find that
radiative dust cleansing by nearby luminous stars may be the explanation for
the peculiar composition of both the Sun and M67-1194, but alternative
explanations are also possible. The chemical similarity between the Sun and
M67-1194 also suggests that the Sun once formed in a cluster like M67.
