Many people plagiarize doctoral dissertations, let alone undergraduate dissertations.
General schools are not very strict. After all, there are internships and job hunting, and students are also very busy. Moreover, the teacher has to bring several students' graduation papers and has no energy to check them carefully.
I think as long as you don't copy everything, you can correct it yourself, and the defense teacher doesn't happen to be your classmate's teacher. I don't think it's that coincidence ~ this is personal experience ~
Good luck ~