Can I get the same result by coping a raw ( same exposure ) file many times and stacking to reduce noise as I would if I used many separate exposures?

No. If you stack copies of the same image, you'll amplify the noise right along with the signal.

Stacking images to reduce noise is an averaging process. It's like a science experiment: you don't measure something just once, you measure it multiple times and average the measurements to reduce the effects of error in the measurement process. Doing what you propose would be like taking just one measurement and writing it down in your lab notebook several times.

When you take an image, you're measuring light intensity at every point on the sensor, and noise is essentially the error in each of those measurements. Because the error is random, it changes from one exposure to the next. For two adjacent photosites, the data recorded across 5 exposures on a 0-255 scale might be:

150, 152
148, 146
144, 145
151, 148
147, 149

Adding those up and dividing by 5, the average value for both photosites is 148. Averaging the values over several exposures smooths out the data, eliminating the noise in each photosite. Even though the individual measurements for the two photosites are a little different, they both have similar averages because they're both recording the same part of the scene.

Now consider what would happen if you stacked multiple copies of the first exposure:

150, 152
150, 152
150, 152
150, 152
150, 152

Now you're not removing any noise at all -- the averages are 150 and 152 respectively. You get exactly the same degree of noise that's present in the image that you used.