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The merging of supermassive black holes (SMBHs) is a direct consequence ofour hierarchical picture of galaxy evolution. It is difficult to track themerging process of SMBHs during mergers of galaxies as SMBHs are naturallydifficult to observe. We want to characterise and confirm the presence of twoindependent active galactic nuclei (AGN) separated by a few kiloparsec in sevenstrongly interacting galaxies previously selected from the Sloan Digital SkySurvey (SDSS) as Seyfert-Seyfert pairs based on emission-line ratiodiagnostics. Optical slit spectra taken with MODS at the Large BinocularTelescope (LBT) are presented to infer the detailed spatial distribution ofoptical emission lines, and their line ratios and AGN signatures with respectto the host galaxies, thereby quantifying the impact of beam smearing and largefibre apertures on the spectra captured by the SDSS. We find that at most twoof the seven targets actually retain a Seyfert-Seyfert dual AGN, whereas theothers may be more likely powered by post-AGB stars in retired galaxies orthrough 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 fluxfrom the primary source in the secondary SDSS fibre which can be more than anorder of magnitude at <3" separations. Previously reported extremely lowX-ray-to-[\ion{O}{iii}] luminosity ratios may be explained by thismisclassification, as can heavily obscured AGN for the primaries. We also findthat the nuclei with younger stellar ages host the primary AGN. Studies ofclose dual AGN selected solely from fibre-based spectroscopy can create severebiases in the sample selection and interpretation of the results. Spatiallyresolved spectroscopy should ideally be used in the future to characterise suchcompact systems together with multi-wavelength follow-up observations.

Revisiting dual AGN candidates with spatially resolved LBT spectroscopy -- The impact of spillover light contamination