  X-ray quasi-periodic eruptions (QPEs) are intense repeating soft X-ray bursts
from the nuclei of nearby galaxies. Their physical origin is still largely
unconstrained, and several theoretical models have been proposed to date. We
present here results from a recent XMM-Newton observation of GSN 069, the
galactic nucleus where QPEs were first discovered. After about two years of
absence, QPEs have reappeared in GSN 069, and we detect two consecutive QPEs
separated by a much shorter recurrence time than ever before. Moreover, their
intensities and peak temperatures are remarkably different, a novel addition to
the QPE phenomenology. We study the QPE spectral properties from all XMM-Newton
observations assuming QPEs to either represent an additional emission component
superimposed on that from the disc, or the transient evolution of the disc
emission itself. In the former scenario, QPEs are consistent with black-body
emission from a region that expands by a factor of 2-3 during the individual
QPE evolution with radius of the order of that of the Sun at QPE peak. In the
alternative non-additive scenario, QPEs originate from a region with an area
6-30 times smaller than the quiescent state X-ray emission, with the smallest
regions corresponding to the hottest and most luminous eruptions. The QPE
reappearance reveals that eruptions are only present below a quiescent
luminosity threshold corresponding to an Eddington ratio of the order of 0.4
for a one million solar masses black hole. The disappearance of QPEs above
threshold is most likely driven by the ratio of QPE to quiescence temperatures
approaching unity at high quiescent luminosity, making QPE detection
challenging, if not impossible, above threshold. We briefly discuss some of the
consequences of our results on the proposed models for the QPE physical origin.
[abridged]
