How can I find an offset between two graphs at every sample point?

I have two sets of Frequency response functions (FRFs). I am comparing their real and imaginary parts separately with each other. I can see that the imaginary parts are almost the match at low frequency, but there is an offset between real parts. I want to know how much is this offset at every known frequency. I am only comparing the FRFs at frequency of 0:500 Hz. I have tried to use interp1() but I get an error that says all the X values must be distinct. I appreciate if anyone could help me with this matter. Thank You