Yeah I cringe a little every time their work shows up in my feed, except their color work is so good tho.
Thats a concern of mine too. What I try to do is base a new (human) character on someone I know in real life, try to find their distinguishing features and exaggerate them. Since in my series, The Working Stiff1, several of my main characters are zombies that's not as big of a concern.
For them I try to think more about how they have deteriorated and decayed. I also try to slightly adjust the hue of green I use for them to give them different skin types. I haven't created model sheets for every character yet but really need to. Sometimes doing a character lineup can help you see them side-by-side and lets you know how much adjusting you might have to do.
This is something that's bugged me for a really long time in my own art, and is something I've been working to fix before I commit to making any kind of story based comics. For the longest time all my characters basically looked the same. They all had this sort of middle aged white guy look to them that was pretty obvious. And I've been sort of putting myself through a sort of challenge to make unique looking characters from my own head which has been a huge help.
I didn't say they'd be pretty characters, just different...
I actually shared a video on that a while back I've been using that I thought was really helpful ( http://forums.tapastic.com/t/iterative-drawing-the-fastest-way-to-improve-or-not-idk/101541 )
Drew some of the male characters in my comic (sorta rushed though), I guess not the same but not that different either? I always have a hard time with nose varieties so I should work on that more.
2nd looks so different without facial hair omgg
When I created my characters I really took time in choosing everything carefully, from face/eye/nose shape to hair type and color.
When I draw them individually and really take my time in the drawing you can actually see the difference. Even with two of my characters that are twins, I made them fraternal twins because I didn't want them to be identical.
But my comic being b/w even for me I find that there is a point when I start making the same faces, specially when drawing the eyes, sometimes I go back and redo everything, others times Im just too lazy and drawing the hair really helps differentiate from them.
I'm not really sure if I've been making this mistake tbh... 😰
I do know that dissent females have this problem, they all look the same... and I've heard that Bleach, in the manga, ended up with this problem...not sure....
I sometimes worry about this but I at least try my best to go for slightly different face/nose/eye ect. shapes... However, sometimes the characters just end up looking kinda off model, making it harder to tell if I "intended" to make them look different in the first place.
I should draw character reference sheets when I get the time...
I guess one big thing about avoiding Same-Face is to not be afraid of making characters ugly. Because let's face it, once your free of trying to make a character attractive looking it becomes easier to give them more, uh, unique features.
I actually start out my facial designs just like this. With the intention in mind if I stripped away everything would the character be distinguished enough on their own. I don't know if I achieve it entirely honestly.... but I'm working on it.
I try to explore facial expressions, not use the same expression for two panels in a row. Use "ugly" faces. Open the mouth more, add more face wrinkles for face muscle creases. ...This is probably why I'll never get into animation because of my inconsistency haha!
Also, I think people focus on trying to draw "attractive" too much. People find inner character more attractive then face, so don't try so hard to make them all "pretty." If someone finds a character of mine visually attractive, I assure you it's not intentionally done, but still quite flattering. (Unless the character was entirely mean to be some sort of vixen then it was intentional.) I focus a lot on eyes and noses, since it is my favorite part to draw. I love drawing big or long "imperfect" noses, ...it can help out a lot.
I tend to give siblings the same eye shape but vary the eyebrows (people forget you can play with the eyebrows and they're so versatile)
I think the 2005, the second 2006, and the 2007 all suffer from the same issue: Nostrils on the same line as the eyes. Makes me think that certain facial proportions are just universal (eg nostrils under eyes).
Edit: I mean unless, you want an alien chibi or something, then I guess more power to you... 
This is definitely a problem I have. I try to vary parts but I'm finding theyall sort of blend into the same one over time. What makes it worse is having gone down the dotted eye route which also becomes an issue in terms of conveying certain emotions.
Seriously considering changing it up for the next issue and trying to keep a level of consistency up.
I find that this starts to become a problem for me when I don't vary my reference. It's useful to use myself as reference especially when drawing comics that are a bit more realistically proportioned but If I'm not careful everybody stars morphing into me, If I'm not careful. Mostly I think it's an issue that's solved very easily as long as its noticed.
That's another thing - I often end up using myself as a reference (webcam photos ect) for a certain expression/pose I want to draw (because let's face it, sometimes googling up the perfect thing you have in mind can be a pain) and sometimes it's like the characters I use that reference for... end up looking kinda like "me" in terms of face shape, build, ect, even if they are supposed to have a different build, face shape ect. Sometimes also gives them a bit too "realistic" proportions as opposed to the more cartoony, sometimes exaggerated/tweaked style (especially for faces) usually used- which just makes them look off-model from their regular design (though, this is kind of an issue with photo refs in general, and in the end it's best to reference from "real life" photos, but still)
Everyone's pretty much nailed the basics of it. I also recommend having charts or references for characters, like so-and-so has these shaped eyes, what's-his-name has a flat nose, whatever. It really helps.
I get a little paranoid about sameface sometimes, but i tried to circumvent it further by giving characters unique expression quirks that set them apart from other characters. Like, one character's expressions were conceptualized around asymmetry, another character has an expression no other character can do, etc.
The latter also really works in strengthening characterization, but thats a different thread for another day.
I try to be very conscious of same face in my work because it is a pet peeve of mine. I think what helps is drawing different people from reference, and really studying what makes a face unique.
I put a lot of effort into designing my main characters and varying their facial features; big things like face shape to subtle things like eyelid shape. I think my biggest problem now is actually getting them to look consistent from panel to panel.