If they're just similar, but not actually copies, or you just designed a character and discovered another character acted/appeared the same; it's fine. In all reality, they are your character, because there is something that will always set them apart. No characters/stories are the same.
You might want to step inside your character. Look at who they are, what they wish to do, and form their personalities and appearance around that.
I had a character who was a bit bland, and another character popped up and they were almost indistinguishable. So, I searched into the first character's life and found that she works for a (let's call it) "police" agency, but her thinking process before this was a bit lax. So, I brought out a sense of order and justice into her life, and this carried on to reason with joining this "police" agency later on.
If you wish to change their personality, because it's bugging you, do what I said earlier. During the revision, rewrite that character's narration and dialogue along the lines that you set for their personality.
If it's appearance, that's a easier fix (but I think your example, is just an example).
It would help to know what character your talking about, and what similarities they share with your character.