] Complete overlap (= identical retention) is a regular feature of experimental chromatography with very different compounds and complex mixtures. It just happens when the selectivity happens to be equal to 1.00. You can have one, two, or three different compounds hidden under a single peak and this usually happens in early eluting peaks in all modes of chromatography of a complex mixture.
Peak overlap is undesirable, but it happens all the time. All you need to do is to change the method.
People have spent a portion of their life in something called as peak deconvolution to find out what is hidden in a single peak. Note how the area adds up.