it is not a matter of does this person know he is sinning, it is a matter of this person is not validly married, and therefore may not approach communion until he rectifies the situation. The Catholic has the obligation to properly form his conscience, and to take active steps to learn what the Catholic Church teaches. I don’t think there is a Catholic on the planet who does not know he should approach his parish priest for advice when he plans to marry. The same people know enough to bring their children in for baptism.
We have 4 sessions, at different times during the year, to discuss marriage issues and make sure people know the teaching, and we have an in depth interview within a month of the time the candidate begins RCIA (required in every diocese, by the way) to uncover marriage issues. If an annulment is needed, we have them sign that they were informed and give them a copy. Yet we still have people when the time comes for confirmation, and I am still asking for proof of marriage, claim we never told them about this.
If any Catholic is in a situation where they are caught in this or any other objectively sinful circumstance, because either they were taught wrong information, or not taught by the person in charge of their catechesis, bear in mind the guilt rests on the person who failed to teach and the pastor who failed to be a proper pastor. There can be no sin without full knowledge. On the other hand, all Catholics are obligated to seek out education on their faith, not just be passive about it.