Any instance of the sin can be absolved. Now, if you’re talking about a family with children where the mother and father remain in the same house for the children’s sake, but are trying to live as brother and sister and occasionally don’t manage it, then there is nothing to stop them simply confessing after they sin, then going back to trying not to, then confessing again should they fail again. Nor should there be.
If, however, you are talking about a couple which knows they are not married, but persists in acting as though they are and has no plans to even try to stop sinning, then the problem is not that a priest can’t absolve any instance of thier sin, but that the state of planning on continuing to sin shows a lack of contrition (which includes the resolve to sin no more, CCC 1450 ish), and would make the confession invalid.
So yeah. Repeated confession of the same sins is perfectly possible. The problem comes when people either refuse to admit that what they are doing is sin, or, knowing this, plan to continue committing them regardless.