  We analyze the screening of static diquark sources in 2-flavor QCD and
compare results with the screening of static quark-antiquark pairs. We show
that a two quark system in a fixed color representations is screened at short
distances like a single quark source in the same color representation whereas
at large distances the two quarks are screened independently. At high
temperatures we observe that the relative strength of the interaction in
diquark and quark-antiquark systems, respectively, obeys Casimir scaling. We
use this result to examine the possible existence of heavy quark-quark bound
states in the high temperature phase of QCD. We find support for the existence
of $bb$ states up to about $2T_c$ while $cc$ states are unlikely to be formed
above $T_c$.
