I think this is the normal behavior of a gaussian blur.
What you see is the blur of the persons extending beyond the actual mask.
If you don't want that you should actually enlarge and feather the mask to mask the blurred area
What people usually do is cut and paste the background on another layer, enable the transparency lock then blur not to have the blur polluted
if you try your solution duplicating the men you will have the same result
So you have to use the transparency lock on the background hence not duplicate your three man like you said but the background with a transparent area were your three man stand and the transparency lock.
I don't know no other way to achieve this result because you need the transparency lock
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This is normal. The transparency lock shown by olh21 will work if the selection is hard , but any softness and you will still get a blur.
The alternative is
a. Select the subject (as you did)
b. Put that selection as a mask on the upper layer (revealing selection)
c. Apply the same selection to the lower layer and use content aware fill
d. Apply gaussian blur to the lower layer
1) Want to thank those who replied. I'm always impressed with the generosity of people in this forum who are so willing to donate their time and expertise to help others. Thanks very much.
2) Now that I think about it: Of course! In blurring the entire image, of course I'm blurring the subject as well as the background. If I then mask the subject, the blur will extend beyond the mask. I missed the obvious. I also learned about transparency lock. I had no idea.
Again, many thanks.