Chemical potential of mixtures of ideal gases and the Gibbs-Duhem equation
The chemical potential of a the two species in your ideal gas mixture are $$\mu_A=\mu^0_A(T)+RT\ln\left(P\frac{n_A}{(n_A+n_B)}\right)$$
$$\mu_B=\mu^0_B(T)+RT\ln\left(P\frac{n_B}{(n_A+n_B)}\right)$$
and the total free energy of the mixture is:
$$G=n_A\mu_A+n_B\mu_B$$So, the chemical potential of A depends on the number of moles of B and the chemical potential of B depends on the number of moles of A.  Do these equations satisfy the Gibbs-Duhem equation?  Try them out and see.