Yes. When you compress the first time with H.264, it throws away visual information and introduces artifacts. When you use and compress that again, it throws away more visual information and introduces additional artifacts (while also attempting to preserve the original flaws!).
This is call "generational loss" and it's similar to making a copy of a copy of a VHS tape.
It's better to work with an "intermediate codec" or "mezzanine codec" during post-production to eliminate or greatly reduce generational loss. Some options include the Lossless output module preset, or other codecs like CineForm, DNxHD or ProRes.