We establish a dimension formula for the unreduced singular instanton homology of dual knots Kp/q⊂Sp/q3(K) for a knot K⊂S3: dimI♯(Sp/q3(K),Kp/q,ω;K)=2q⋅rK(K)+2∣p−q⋅νK♯(K)∣forp/q=νK♯(K),where ω⊂S3\K is any unoriented 1-submanifold as the bundle set, rK(K) and νK♯(K) are integers from the dimension formula of I♯(Sp/q3(K);K) for a field K defined by Li and the author. In particular, when K is the two-element field F2, the reduced singular instanton homology satisfiesdimI♮(Sp/q3(K),Kp/q,ω;F2)=dimI♯(Sp/q3(K);F2)forp/q=νF2♯(K).As an application, for a determinant-one knot K⊂S3 other than the unknot and the torus knots T2,3,T2,5 and a rational p/q∈(0,6) with p odd prime power, the surgery manifold Yp/2q(K) is not SU(2)-abelian for the double branched cover Y=Σ(S3,K) and the preimage K⊂Y of K. We also obtain non-abelian results for SU(2) representations of the knot complement that send the curves of some fixed slope in (0,6) to traceless elements.