  Double white dwarf (double-WD) binaries may merge within a Hubble time and
produce high-mass WDs. Compared to other high-mass WDs, the double-WD merger
products have higher velocity dispersion because they are older. With the power
of Gaia data, we show strong evidence for double-WD merger products among
high-mass WDs by analyzing the transverse-velocity distribution of more than a
thousand high-mass WDs (0.8--1.3 $M_\odot$). We estimate that the fraction of
double-WD merger products in our sample is about 20 %. We also obtain a precise
double-WD merger rate and its mass dependence. Our merger rate estimates are
close to binary population synthesis results and support the idea that
double-WD mergers may contribute to a significant fraction of type Ia
supernovae.
