  We analyze the effect of weak disorder on the competition between
antiferromagnetic order and superconductivity in a model for iron-based
superconductors. Under the assumption of an approximate particle-hole symmetry
we show that conventional $s^{++}$ superconductivity cannot be realized in the
case of coexisting magnetic and superconductive orders, observed experimentally
at intermediate doping levels. This result holds for arbitrary impurity
concentrations, and, in particular, in the clean limit. The inclusion of
disorder further amplifies the phase competition between itinerant
antiferromagnetism and conventional superconductivity. In addition, we analyze
the effect of disorder on the characteristic length scales of the two order
parameters, and find that in a disordered sample the staggered moment
fluctuates on shorter scales than the superconductive order parameter.
