Need help with strange scaling behavior on my torso model; following a blender video I am brand new, and I am following a blender video by Sebastian Lague, and I just want to figure out why i am not able to properly scale the selected edge of my torso model, as he does in the video.


  The vertices / edge i have selected before
  scaling (I use alt+right-click or
  alt+shift+right-click, just as in the video)
  
  What the issue looks likes when i try to scale it with the S or alt-S
  hotkeys
  
  What it should look like


What the heck is going on? If this isn't normal behavior, I seem to have accidentally created double vertices at the same points or something? If this behavior is normal, i must not be selecting the edge / vertices properly?

Thanks in advance and happy holidays!
You have a different number of loop cuts than the example - were you embellishing the design, or had you made some errors in following the example?  From my own experiences, making errors and then correcting them during a tutorial led to some weird results and it was a while before I learnt to remove duplicates.

To check your theory about double vertices, select your entire mesh (A followed by A again should do the trick from the view in the images) and then select remove duplicates from your left hand menu. Off the top of my head, I can't remember if it is T or N to display this menu from the view that you have in your images - one will bring up the right properties menu and the other will bring up the left menu options. 

Are you also trying to scale along the right axes? S-Y will ensure it is locked on the y-axis as noted in the video. Selecting Shift-Y instead of Y will result in the scaling happening along the X- and Z-axes. Can't judge from your static screen shots.