Often both doctor and patient feel asking for a second opinion is a sign of distrust. But two head are better than one - especially if you have some something a bit out of the ordinary; that isn't getting better like it should; is calling for some drastic measures like surgery or chemotherapy.
It is often a good move that I encourage any emancipated patient to seriously consider in Managing Your Doctor. And a sensible doctor is happy to share the responsibility.
An article by Kathleen Hoffman, in Medivizor (https://medivizor.com/blog/2016/02/08/eliminate-doubts-getting-second-opinion/) gives good advice on the subject.