Wrong repetition of object
The doctor I know him very well.
I know the doctor very well.
In ihe sentence given, the worcs doctor and him denote one and the same object Therefore, use either doctor or him, but not both in the same sentence-. In general we don't put the object before the verb so the word order in the doctor I know him very well is also wrong.
Tips to success in MAT Exam
Practice a lot
To excel in this exam it is important to practice a lot. One of the most important ways to crack GMAT is take as many mock tests as possible. You can take mock tests from this site. It will also give you the idea that how much time you are taking to solve a particular section. While practicing the mock papers you can also do time management to attempt all the sections of the exam.