Some of my stories sound like other, some do not. It depends on the story, and who the protagonist is.
Whether it's a bad thing depends primarily on how good the stories are. If they're good enough to sell, no problem. Some writers use the same main characters in story after story for years. If you'rewriting a series character, sounding the same is good.
That said, most writers like to mix things up. Th first step is to stop using main characters who are the same as they were in the last story, or the story before that. Use young people, old people, men, women, children, etc. Use people in different professions, from different parts of the country, on and on. No two people are the same, and no two main characters should sound the same, either. Even two characters who are both professors at Harvard, or who are both police officers, should not sound exactly the same. We each have our own vocabulary, our own sense of humor, our own speech mannerisms, and you have to bring these out. It takes a lot of thought, and it takes knowing who the character is.
As for stories, use different settings, vary the humor, change up the plots, take left turns and surprise the reader. Use characters who are as different as the setting, etc.