PI wants to add a new author who I think doesn't deserve to be added
I understand what you are going through. Your best shot is to discuss this with your advisor! But, you need to consider the following;


I do not know your field (although you have used computer-scinece as a tag), but in some fields, conference papers are not that important. I'm not saying that adding a new author should be fine! All what I'm saying that as long as your name is 1st, or 2nd author, you should be fine. (Personally, I would not be fine if the new guy's name came before mine in this specific scenario). If conference papers are weighted heavily in your field (i.e., computer science), that is a different story and you may need to talk to your advisor for clarifications. 
It also depends on the topic, if the topic is specific (i.e., cutting edge technology) rather than general (i.e., literature review), then adding an additional author may cause irritation on your part and this need to be addressed with the advisor (again).
You do not know the whole story! What is the point of adding a new guy? Why redo your experiments? If it is due to funding/proposal/politics, you need to consider the bigger picture. 
In general, you need to learn to cooperate! The sooner the better. You do not end up working with the people you like or the same people over and over again! My advisor would make two students work on a topic that is remotely related to both of them just to teach them how to cooperate. Then, he publishes two papers (where he switches the students names between 1st and 2nd). For instance, one paper would be on an experimental approach to solve a problem where student A is the first author. the 2nd paper will be on analytical/numerical approach to be validated against the experimental work where student B is the first author. 


P.S. 
When I discussed collaboration in point 4, I do not mean that your advisor should add somebody in a paper you have been working on for sometime (or has an outcome of your direct research/thesis). But, rather for "networking", "motivation", "learning" and "brainstorming" aspects.