I got married 2 years ago and had an interview for my first green card. Next year I am applying for the 2nd one but the officer who interviewed us said we won't need to go to an interview again. We only need to send our documents with this I-751 form. Is this a normal situation? Why they don't need a second interview?

Submit I-751 with all supported documents... USCIS will notify you whether you need to go in person for an interview.

If you have solid evidences, you may receive green card in the mail, no interview. If they are not convinced that your relationship is real, you and your spouse will be scheduled for an interview.