“Two of my characters have incredibly similar names, but I love them both dearly. Do you think it’s a problem for them to have similar names? I’m talking both first and last names.”
Yes and no. First, is it on purpose? Is there a reason they should have similar names? There are instances in stories where the point is that these characters are meant to be incredibly similar and easy to mix up.
You can also mix up characters who don’t have similar names. Reading Harry Potter, I mixed up Fred and George constantly. Their names are not similar but they do almost everything together, they talk very similarly and tend to respond to things not as individuals but as a pair. Had they had similar sounding names, they would have only been even more interchangeable. Later in the series, slight differences do emerge and in those cases, the very different sounding names help differentiate them.
There’s also examples like in The Sound and the Fury, Quentin Compson and Miss Quentin. Miss Quentin was named after her uncle, Quentin Compson. The “Miss” exists solely to tell them apart. The novel jumps in time and skips around so this does get a little confusing, especially early on. Though they have the same name, these characters are very different people. They do have some things in common and the name prompts the reader to search for more similarities.
There isn’t necessarily a direct right or wrong answer here, but a name is purposeful. Make sure that your names work with each other and aid your story.