  In this paper we present and study the numerical duplication of a numerical
semigroup, a construction that, starting with a numerical semigroup $S$ and a
semigroup ideal $E\subseteq S$, produces a new numerical semigroup, denoted by
$S\Join^b\E$ (where $b$ is any odd integer belonging to $S$), such that
$S=(S\Join^b\E)/2$. In particular, we characterize the ideals $E$ such that
$S\Join^bE$ is almost symmetric and we determine its type.
