  The Initial-Final Mass-Relation (IFMR) maps the masses of main sequence stars
to their white dwarf descendants. The most common approach to measure the IFMR
has been to use white dwarfs in clusters. However, it has been shown that wide
double white dwarfs can also be used to measure the IFMR using a Bayesian
approach. We have observed a large sample of 90 Gaia double white dwarfs using
FORS2 on the VLT. Considering 52 DA+DA, DA+DC, and DC+DC pairs, we applied our
extended Bayesian framework to probe the IFMR in exquisite detail. Our
monotonic IFMR is well constrained by our observations for initial masses of
1-5 Msun, with the range 1-4 Msun mostly constrained to a precision of 0.03
Msun or better. We add an important extension to the framework, using a
Bayesian mixture-model to determine the IFMR robustly in the presence of
systems departing from single star evolution. We find a large but uncertain
outlier fraction of 59$\pm$21 percent, with outlier systems requiring an
additional $0.70^{+0.40}_{-0.22}$ Gyr uncertainty in their cooling age
differences. However, we find that this fraction is dominated by a few systems
with massive components near 0.9 Msun, where we are most sensitive to outliers,
but are also able to establish four systems as merger candidates
