Conference presentation of a weak paper to an expert audience. As a part of my research project, I am almost finished a first paper. I was proud of myself because of the contributions: I was able to take some ideas that has been used in the field for some years now, and I provided a unified point of view. However, as it turned out, some of those results had been already obtained before. Nevertheless, this results seems to be unknown yet in the community of the field.

A very important conference is coming up, in which the main leaders on the topic will be present. My supervisor wants me to give a talk about my results in this event. Given the fact that a few of these results had been already obtained (and of course, I am not taking credit for this), the paper with only my contributions (discarding what was done before) is not a strong paper. Although my supervisor thinks it is OK, I am afraid that giving this talk in front of these experts will result in them not taking me seriously as a researcher in the future. What are your thoughts on that?
Answer inspired by @Roland's comment.

First of all, have a little faith in yourself, your supervisor seems to believe your work is good enough and so should you.

Otherwise, if you really do have some doubts about the relevance of the paper you can stress the fact that these results seem to be unknown in the community.

Raise awareness on the fact that your project is interesting and that some work and results have already been made, yet seem to have passed under the radar of many.