I often experience the same visuals and sounds on the same drugs. I've never had the same exact trip though. I always figured that different psychedelics cause a different set of hallucinations in people (though not necessarily the same between different individuals). For example, on DOC I always see almost a sort of clear goo overlaying everything which contains morphing and fractals in a particular way. Always the same colors, always the same character to the visuals. Clouds always morph the same way, etc. On DiPT, the sound alterations are always precisely the same, at the same dosage level at least (DiPT produces different levels of distortion at different dosages). My first trips were different in some ways but once I got familiar with tripping I have always found the trips to have the same character every time, for most drugs. The content (emotionally, mentally, etc) can be very different though between individual trips.