Duplicate stitch for embroidering and sewing together

Dear community,
How does the duplicate stitch work for embroidering and sewing together?

Does anyone else have anything to add?

I like to use the Kitchener stitch when sewing together horizontally knitted hats, or also infinity scarves, when the seam is meant to be invisible.
With a little (or a bit more) skill, you can replicate a knit row, a purl row, and even a garter stitch row. 

Sometimes patterns say to sew shoulder seams together using the Kitchener stitch. But that would make them just as stretchy as regular knitting. That’s why I would never do that—a shoulder seam holds the entire garment together. So it needs to be sturdy and shouldn’t be able to stretch all the way to the elbow.