Question about adding co-first author The subject of authorship is always a bit challenging, but here's a dilemma: Person A recently graduated from a PhD program in Molecular, Cell Biology. Prior to graduating, A submitted his manuscript for publication listing A as first author, one middle author and the PI as corresponding author. A had conceived of the project, spent 3+ years of full time work conducting the experiments, wrote the manuscript, did all the analysis and put together all figures. The reviewer comments came back as a revise and resubmit. At around the same time A was told by the PI that he needed to graduate and be off the grants for funding reasons, and that student B, a third year in the same lab, would be able to help with increasing replicates and the few experiments required by reviewers. A also continued to analyze data and offered to help with experiments in the lab on a volunteer basis, but the PI said that wouldn't be necessary as B could take care of it. B conducted some replicates at the rate of 1-2 experiments per month for 3 months (he was juggling several other projects) until the revisions were due. Two nights before the revised manuscript was due the PI said that B should be added as a co-first author, indicating that A and B have contributed equally to the work. My question is this: Is this normal in academia? Is it something that should happen and is no big deal, as indicated by the PI? Also, what can A do given that he has already graduated and will be starting a post-doc and will need recommendations from the PI in the future?
Sadly enough it is pretty common. Not universal, but more common in certain fields than others. You can fight it, but I recommend that you don't. 

The reasons for letting it go is that you have finished and have a position (I assume). Over the course of your career, this sharing of first authorship on one paper will mean little, even if it is based on your dissertation. But the good will of your advisor, and possibly the other student can pay dividends in the future. 

Your dissertation work is your first work, but, hopefully, not your best ever. And having the old professor being willing to work with you to advance your career and so breaking ties now is probably a sub-optimal move. 

Yes, fairly normal. Questionable ethics. But optimize a path for yourself, and a single paper is only a small part of it. Yes, I know it is disheartening. But the long view is worth considering. 

And, I've assumed you are A, here, hence the wording.