If this was true, no one would bother getting a second opinion of medical advice from a doctor. We'd accept the advice from the first expert.
You might note that in my reply, I said "The
ones with the most knowledge and experience in the relevant field." —
ones, plural, not singular.
That's how science, including doctors and medical science, works. Each has their individual work, but they also work together, often in teams, in order to broaden their understanding and expand the range of knowledge being applied. As they all have human limitations, no one person can absorb all of the available knowledge and data in science, medicine, or particular field and they routinely seek out knowledge, opinions, and advice from others within their field of expertise and from other fields as well. They routinely engage in consultations with each other. The more serious or complex a disease or condition, the more likely those additional consultations become.
This is why often, a doctor making a diagnosis or recommending a treatment plan will suggest a second opinion or consult with other colleagues themself. It's done to insure that before a consensus is reached on diagnosis or treatment that someone else with similar expertise, but their own collection of knowledge and experience, has come to the same conclusions and recommendations. If there are differences of opinion or questions raised, it indicates a need for further investigation. It's good professional practice, good advice, good medicine, and good science. It's also why science journals engage in peer review prior to publishing articles. Scientists and doctors check each others work for omissions or errors in data, including human error, looking for explanations and recommendations based on the best available evidence. That's how science, including medical science, routinely works.