  In situ measurements from spacecraft typically provide a time series at a
single location through coronal mass ejections (CMEs) and they have been one of
the main methods to investigate CMEs. CME properties derived from these in situ
measurements are affected by temporal changes that occur as the CME passes over
the spacecraft, such as radial expansion and ageing, as well as spatial
variations within a CME. This study uses multi-spacecraft measurements of the
same CME at close separations to investigate both the spatial variability (how
different a CME profile is when probed by two spacecraft close to each other)
and the so-called ageing effect (the effect of the time evolution on in situ
properties). We compile a database of 19 events from the past four decades
measured by two spacecraft with a radial separation <0.2 au and an angular
separation <10{\deg}. We find that the average magnetic field strength measured
by the two spacecraft differs by 18% of the typical average value, which
highlights non-negligible spatial or temporal variations. For one particular
event, measurements taken by the two spacecraft allow us to quantify and
significantly reduce the ageing effect to estimate the asymmetry of the
magnetic field strength profile. This study reveals that single-spacecraft time
series near 1 au can be strongly affected by ageing and that correcting for
self-similar expansion does not capture the whole ageing effect.
