  Context: White dwarfs (WDs) are important and abundant tools to study the
structure and evolution of the Galactic environment. However, the multiplicity
of WD progenitors is generally neglected. Specifically, a merger in a binary
system can lead to a single WD, which could result in wrongly inferred
quantities if only single stellar evolution (SSE) is considered. These mergers
are linked to transients such as luminous red novae and Type Ia supernovae.
Aims: We investigate the impact of binary evolution (BE) upon observable single
WDs, and compare their properties to WDs formed through SSE. We assess the
evolutionary channels and the age and mass distributions of the resulting
single Wds. Methods: We employed \texttt{SeBa} to model the evolution of single
star and binary populations. We synthesised the observable single WD population
within $100$ pc, including cooling and observational selection effects.
Additionally, we constructed models with different evolution and primordial
population properties to study the effects on the properties of the resulting
single WDs.
  Results: White dwarfs from binary mergers make up about $10-30\%$ of all
observable single WDs and $30-50\%$ of massive WDs. On average, individual WDs
take
  $3.1-5$ times longer to form through BE than SE, and so appear ${\sim} 1$ Gyr
younger than they are if BE is ignored. In all models, the effect of mergers on
the age distribution is clearly noticeable. The median age typically increases
by $85-430$ Myr and $200-390$ Myr for massive WDs. Although abundant, we do not
find evidence that WDs from mergers significantly alter the shape of the WD
mass distribution. Conclusions: Assuming SSE for inferring properties of single
WDs gives rise to intrinsic errors as single WDs can also be formed following a
binary merger. Strategies for mitigating the effect of mergers on the WD age
distributions are discussed.

Solar and Stellar Astrophysics