@swnndn Time travel is paradoxical, but there are solutions which allow for this. The simplest one is that every time you time travel you enter an alternative timeline which is entirely unrelated to the one you came from. You can then set up a scenario where it seems like Jane / John are their own parents, but they are actually different versions of Jane / John from different timelines that give birth to a new version of Jane / John.
This movie does give the impression that it's in a consistent time loop, but I don't remember it explicitly stating that there are only one timeline. Minute Physics on YouTube has a video that explains a theoretical solution for this as well, but its a bit more complicated to wrap your head around.