At first glance, I feel like that's kind of a weird question, because what's the point of creating multiple characters if they all act the same...? ^^; I would think some level of uniqueness would be the default choice, not something you have to 'make sure' of...
But then I guess the issue here is that people have different motivations for creating characters...for instance, an amateur might create Character B for the sole purpose of being Character A's best friend, and B's personality or what they add to the story is secondary, if not completely irrelevant.
Or a more advanced (but still misguided) writer might differentiate characters ONLY through character flaws, so as the story goes on and they overcome these flaws, they devolve into a bland, uniform hero hive-mind (* cough* RWBY * cough*).
But I still feel like you could simply...just not do that? ^^; Like, if you have enough emotional intelligence to recognize that people have different personalities, you could just...give your characters different personalities...
Like, let's say Character A is shy and reserved...so let's make their best friend Character B also reserved, but NOT shy. They are as quiet as A (a common thread to build their friendship on!), but only because they prefer not to interact with people, not because of any social anxieties. They are more than willing to take action and defend A when necessary (now A and B have a dynamic!).
On their adventure, they go to a new land and meet Character C...to highlight that this is a novel experience, let's make C really different from the first two and have them be outgoing and loud. Maybe B will dislike C, but the more sensitive A will see their good qualities. Maybe C will tease A at first, prompting them to start standing up for themselves (character development!). The clash of personalities is an opportunity to do a lot of interesting interactions, establish new dynamics, and flesh out our characters.
Lots of stuff will happen, and eventually A, B, and C, will have become friends, and stronger characters as well. But we ain't done yet-- they still need to meet Character D.
Last time we highlighted the new character's personality by giving them something different from the established group: quiet characters encounter a loud one. But now that the established group has more variety, we have to take a different approach if we want to do this again. What do the members of Team ABC all have in common right now...?
Well...they're all nice people. So let's make Character D a mean person!
Now we have a real writing challenge~. The process of Team ABC's friendship may have been eventful, but it could easily be called inevitable since they're all kind, decent people working towards the same goal...no matter what, they'd probably end up seeing eye-to-eye eventually.
Character D, however, may not even want to help ABC at first. Maybe they're even trying to hurt ABC and need to be convinced to switch sides (the classic redemption arc!).
Maybe D's presence in the group will cause friction between ABC as they disagree on how to deal with it-- B might be against even having them around, and C might think they can 'reform' them with enough time. A might not know what to think, and become stressed from watching their oldest friend and newest friend fight and argue.
And of course, D can have thoughts on this, too-- maybe B's distrust offends them and they act out. Maybe B is right and the fact that they're deceiving C makes them feel a little guilty. Maybe they gravitate to A for an 'objective' opinion, and A and D grow together as characters. Maybe all of these things at once. ^^;
TL;DR Just give each new character some noticeable difference from the characters that come before them, and then USE that difference (very important) to develop them all as a cast.