The ground state of carbon mono-oxide with increased bond length hasan unusual electron correlation which is never obtained by a singleunrestricted Hartree-Fock (UHF) wave function. Dissociating CO hasthree UHF states which converge to the correct dissociation limit, twoof them are of axial spin density wave (ASDW) type and another is ofaxial spin wave (ASW) type. One of them, ASDW_1, bifurcates from therestricted Hartree-Fock (RHF) ground state, but the other two, ASDW_2and ASW, which have lower energies than ASDW_1, do notbifurcate from ASDW_1 and RHF but cross withRHF. Configuration analysis shows that none of them contains all theessential configurations in the ground state wave function. However, asuperposition of projected ASDW_1 and ASDW_2(or ASW)wave functions gives a good approximation to the ground state, showingthat it is in resonance of the two UHF states, namely two differentkinds of collective correlation are resonating. The resonating UHFground state is shown to originate from the double well structure ofthe UHF variational energy surface which yields two UHF states withnearly degenerate energies.