  The merging of supermassive black holes (SMBHs) is a direct consequence of
our hierarchical picture of galaxy evolution. It is difficult to track the
merging process of SMBHs during mergers of galaxies as SMBHs are naturally
difficult to observe. We want to characterise and confirm the presence of two
independent active galactic nuclei (AGN) separated by a few kiloparsec in seven
strongly interacting galaxies previously selected from the Sloan Digital Sky
Survey (SDSS) as Seyfert-Seyfert pairs based on emission-line ratio
diagnostics. Optical slit spectra taken with MODS at the Large Binocular
Telescope (LBT) are presented to infer the detailed spatial distribution of
optical emission lines, and their line ratios and AGN signatures with respect
to the host galaxies, thereby quantifying the impact of beam smearing and large
fibre apertures on the spectra captured by the SDSS. We find that at most two
of the seven targets actually retain a Seyfert-Seyfert dual AGN, whereas the
others may be more likely powered by post-AGB stars in retired galaxies or
through shocks in the ISM based on spatially resolved optical line diagnostics.
The major cause of this discrepancy is a bias caused by the spillover of flux
from the primary source in the secondary SDSS fibre which can be more than an
order of magnitude at <3" separations. Previously reported extremely low
X-ray-to-[\ion{O}{iii}] luminosity ratios may be explained by this
misclassification, as can heavily obscured AGN for the primaries. We also find
that the nuclei with younger stellar ages host the primary AGN. Studies of
close dual AGN selected solely from fibre-based spectroscopy can create severe
biases in the sample selection and interpretation of the results. Spatially
resolved spectroscopy should ideally be used in the future to characterise such
compact systems together with multi-wavelength follow-up observations.
