When a person becomes bar mitva during the year, 98% of the time the community has already read 1 or even 50 weekly portions.
- Does the bar mitzva need to make up those portions?
- Would it matter if the bar mitzva had previously learned these portions when he was underage?
The law of completing the portion with the community is brought in Brachos 8a/b followed by the story of R' Bibi bar Abaye who figured on completing it on the eve of Yom Kipur. This would have been acceptable but for a technicality that he should be eating. Does, then, the completion for all previous portions apply at any point in the year as opposed to a weekly basis?
And if so, would that include all portions of the community, or only those read since he became bar mitzva?
If he does have an obligation on portions predating the bar mitzva, could he include portions that he read underage since both times the mitzva was only rabbinic, or do we make a distinction between a single rabbinic and a double rabbinic (The mitzva is rabbinic and his underage obligation to do rabbinic mitzvos is only rabbinic).