  Searches for lepton number violating $K^+\to\pi^-e^+e^+$ and
$K^+\to\pi^-\pi^0e^+e^+$ decays have been performed using the complete dataset
collected by the NA62 experiment at CERN in 2016-2018. Upper limits of
$5.3\times 10^{-11}$ and $8.5\times 10^{-10}$ are obtained on the decay
branching fractions at 90% confidence level. The former result improves by a
factor of four over the previous best limit, while the latter result represents
the first limit on the $K^+\to\pi^-\pi^0e^+e^+$ decay rate.
