When I say loading, I'm referring to the state they are in before actually playing them. In this case, it is the sound being put in the variable(as you say for the DLL), which can only play one at a time, as the DLL interrupts. So if you wanted a second one playing over, you'd have to load a second one into another variable.
Normally, there is one version of a sound loaded into the IDE, and in a sense it gets duplicated each time it is played(there is more to it then that, including things you stream from the hard drive). It isn't necessarily actually duplicated, though the actual sound buffers would contain duplicated data for the small bytes of sound data that is actually in the buffer. Each sound that is playing has a buffer, but I don't know how many bytes they use these days for those. So technically, the single sound would get bits and pieces of it duplicated in those buffers, even if you only have one of them at a given time.
Like I said, the better way which is used by everybody I know is to simply not play the sounds if they are already playing. For most people, just making sure you can only shoot the bullets some amount per second is enough, as the sounds don't overlap as much. You also want to only play one sound even if the single act of shooting creates multiple bullets, meaning you don't play the sound when you create a bullet, rather on the object that handles the shooting. So a single shot could create 20 bullets, but you only get one sound. Then, if you limit it to only 8 shots per second, and the sound is about an 8th second long, you don't get any overlap at all, without having to do any special handling of it. If you have multiple objects using the same effect though, you might need to do things differently, which is where I suggested having a little extra system that tracks how long it has been since the sound last played and skips that playing of it. All you need is an extra function really, which uses global variables for those sounds that need it. You could use a ds_map with the sound id as keys to store the time played data for example. This is why my audio system has a thing that handles it, but since you are seemingly not going to upgrade it won't work for you as it needs the later GMS2 versions to work.