I submitted a paper last month. And the result came out yesterday. Two reviewers gave me high scores while the other one said he saw part of my paper three months ago. In fact, three months ago, I drafted part of my paper and submitted it to a workshop because I want to own a paper which could help me apply for Ph.D. Now this reviewer gave me only 1 for originality. But the paper for a workshop should not be the "record of publication" according to the rules of conference. Should I argue with the chairs of the conference? This is a top conference in our field so I really want this paper published.
If you submitted a month ago and already got rejected, was the new venue truly a "conference" or another workshop? And, as @Pete L. Clark said, was it published before or only submitted and rejected?
If published in a workshop (or accepted for publication), substantial portions can't be reused in another workshop. Portions can be reused in a conference IMHO, but as @Pete said, you have to disclose and cite. If not, it's self-plagiarism.
If the new venue is a workshop, then it can't be reused that much unless the earlier one was rejected. In which case the reviewer shouldn't hold the earlier publication against you, unless they meant to say "I saw this before and it was rejected because it had flaws, and now I'm seeing it again".