I honestly struggle a bit with same face syndrome as well. Your heads have solid form which is awesome to see. To demonstrate how to avoid same face syndrome I started with a drawing based off one of your heads and slightly modified the proportions of the mouth, eyes and chin. I then took that drawing and continued modifying the size, shape and placement of features to get a 1.old man 2. woman 3. heavier set fellow. What it comes down to is avoid drawing the same set of eyes, ears, nose, mouths etc all the time. Challenge yourself to try a new head shape or lip shape. Slightly narrower nose or eyes. It actually gets quite fun seeing how far you can push things. Don't be afraid to break the rules too. I hope this helps.