I received the hep b vaccine as a kid, but my new employer offers it as well. At 20 years old should I get it again or just say I don't need it? does the vaccine ever stop being effective?
It should show that you got your 3 step series as a kid on your immunization record. If you can show proof that you got it as a kid (i.e. your immunization record) you'll be fine. If you can't show proof, your company may wither have you get the series or sign a declination form saying that you don't want the vaccination and the company/ dept is not liable if you get Hep B for example.
I just recently went through the process of applying for medic school and had to get titers for MMR, Hep B, and Varicella, unfortunately my Hep B titers were below the normal range so I have to do the 3 step series all over again (whoopee me). So in regards to your question just because you got a vaccine doesn't mean you have antibodies up to par in your body. Someone with more knowledge than me could give you a more accurate answer than I did.
If you got the series and your employer is offering, you might want to consider doing a titer to see if your immune response is adequate. If it's not, then do the series...