  A formula of the $D$-$D$ correlation function is derived. The deuterons are
treated either as elementary particles or as neutron-proton bound states. In
the first case the deuterons are directly emitted from a source and in the
second one the deuteron formation is a final-state process simultaneous with a
generation of $D$-$D$ correlation. The source radius of deuterons formed due to
final-state interactions is bigger by the factor of $\sqrt{2}$ than that of
directly emitted deuterons. To check how sizable is the effect we compute the
$D$-$D$ correlation function taking into the Bose-Einstein statistics of
deuterons, the $s-$wave scattering and the Coulomb repulsion. The correlation
function is shown to be sensitive to the source radius for sufficiently small
sources. Otherwise the correlation function is dominated by the Coulomb
repulsion and weakly depends on the source radius. Measurements which can make
use of our finding are discussed.
