  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 due to strong interaction and the Coulomb
repulsion. The correlation function is shown to be sensitive to the source
radius for sources which are sufficiently small with RMS radii smaller than 3.5
fm. 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.
