  The distribution of mass on galaxy cluster scales is an important test of
structure formation scenarios, providing constraints on the nature of dark
matter itself. Several techniques have been used to probe the mass
distributions of clusters, sometimes yielding results which are discrepant, or
at odds with clusters formed in simulations - for example giving NFW
concentration parameters much higher than expected in the standard CDM model.
In addition, the velocity fields of some well studied galaxy clusters reveal
the presence of several structures close to the line-of-sight, often not
dynamically bound to the cluster itself. We investigate what impact such
neighbouring but unbound massive structures would have on the determination of
cluster profiles using weak gravitational lensing. Depending on its
concentration and mass ratio to the primary halo, one secondary halo close to
the line-of-sight can cause the estimated NFW concentration parameter to be
significantly higher than that of the primary halo, and also cause the
estimated mass to be biased high. Although it is difficult to envisage how this
mechanism alone could yield concentrations as high as reported for some
clusters, multiple haloes close to the line-of-sight, such as in the case of
Abell 1689, can substantially increase the concentration parameter estimate.
Together with the fact that clusters are triaxial, and that including baryonic
physics also leads to an increase in the concentration of a dark matter halo,
the tension between observations and the standard CDM model is eased. If the
alignment with the secondary structure is imprecise, then the estimated
concentration parameter can also be even lower than that of the primary halo,
reinforcing the importance of identifying structures in cluster fields.
