If it is that much of a "hot topic" then it isn't unusual to throw more person-hours at it. If it is that hot you might derive considerable advantage from getting your work in under deadline, if that is all possible, because you would go into your Masters defense with a good publication already acquired. And perhaps two, since you'll have four months to write another one. So that's a carrot.
In all cases everyone who materially contributes to the solution can legitimately expect to be a co-author on the paper. You might feel that this is a bit of a stick.
However, in all cases your Masters thesis will contain the work that you did, they can't take that way from you. But you can't claim others' work as your own, so if the pertinence of your own work lies in its contribution to a joint paper, you must explain that very carefully in the thesis. For example, you can claim in the thesis that you established effect X, which is a contribution because when combined with effect Y it demonstrates mechanism Z. It is then up to your thesis committee to decide whether this is enough of a contribution.
You will have a lot to gain in sharing your concerns with your advisor and getting some ground rules laid out. For example, if there is a joint paper, am I first author? Will I be excluded from publication if I don't get all of the necessary results by November? What part of this work can you include in your Masters thesis? Don't be afraid to state your worst fears, you need to know the answers and if your advisor is willing to provide assurances it is important to get them.
Finally, if this discussion is oral rather than through e-mail, don't forget to send a follow-up e-mail documenting your understanding of your agreement. If your advisor agrees to your summary, great; if she doesn't respond then your version stands; if she repudiates it then your discussion isn't finished.