Basic Trope: Two characters start out with pretty similar personalities, but later develop their own distinct personalities.
Straight: Bob's two friends Alice and Carol start out as two pretty generic teenage girls with similar personalities, but later on Alice starts becoming more energetic and tomboyish while Carol becomes more of a calm Shrinking Violet.
Justified:Alice and Carol have been friends since early childhood with similar interests, but as teenagers are trying to find their own identities and have not been spending as much time together as before.
Inverted: Alice and Carol originally were completely different characters, but over time they both seemed to become more alike until they were both identical.
Alice and Carol part ways after becoming so different and make new friends. However, they miss each other and go back to being identical in order to be friends again.
Alice and Carol become major characters, but are still very similar to each other.
Double Subverted: However, they keep some of their new traits in their friendship, so that they're still different.
Zig Zagged: ???
Averted: Alice and Carol remain very similar for the duration of their time on the show. Alternately, none of the characters resemble each other that closely in the first place.
Enforced: Alice and Carol were originally not supposed to be main characters, but because they unexpectedly became popular with the fans they decided to expand their roles and therefore had to give them their own distinct personalities.
Lampshaded: Bob says, "I couldn't tell you two apart last year, but now you're completely different from each other!"
Invoked: Bob interacts more with both Alice and Carol, hoping to learn more about them so their personalities become different.
Defied: Bob is afraid that Alice or Carol would be a worse person if they were different from each other, so he avoids interacting with them in order to avoid their becoming different.
Discussed: "You girls may feel like you're similar now, but wait a year and you'll probably be polar opposites."
Conversed: "I can't believe how well this show has managed to make Alice and Carol into different characters. They were so similar at first."
Deconstructed: As both characters begin to change, this is noted by Bob and both Alice and Carol begin to loathe each other for being so similar, speeding up the process until both are miserable and nothing like the way they were.
Reconstructed: But they all find a way to get better about their problems, all culminating in them accepting their similarities.