In general, the industry standard practice for commercially released albums is to have the beat fully multi tracked out (ie kick on its own track, snare on its own track, etc..) Along with the vocal fully multi tracked out within the same project, where every element of the song is mixed together inside one project by a mixing engineer. After mixing, typically the multitrack project is summed together (like as one wav file) and that single file is processed by a mastering engineer.
So basically, mastering occurs once on a single file that had previously been properly mixed together.
With that said, now that most of the tools of the trade have made it into the hands of hobbyist and the general public, people tend to do whatever works for them based on the limitations they have on time, money and know how.
So if you like to send the beat out as one file, apply some process you think of as mastering, then record yourself over that, then sum those together and apply a second process you think of as mastering, no one could tell you it's wrong if you like the results you get.
There's no more rules!!!!