  The NA62 experiment at CERN reports a search for the lepton number violating
decays $K^+\to\pi^-e^+e^+$ and $K^+\to\pi^-\mu^+\mu^+$ using a data sample
collected in 2017. No signals are observed, and upper limits on the branching
fractions of these decays of $2.2\times 10^{-10}$ and $4.2\times 10^{-11}$ are
obtained, respectively, at 90% confidence level. These upper limits improve on
previously reported measurements by factors of 3 and 2, respectively.
