In this game, when you land on the hospital space, you must pay half your salary to the doctor or to the bank.
There is no further clarification about this issue. So what if there is more than one doctor playing? they divide the amount received?
From the games instructions found here:
When completing College, you do not draw an Experience Card; instead you score an automatic salary increase of $20,000, and select a degree as you leave. If you are the first player to pass through College, select any degree from the specialized degrees offered, and check it in the "Education" column of your score sheet. However, each degree may be awarded only once. Thus, if you are the second player to graduate from College, and the first graduate selected the Science Degree, your choice is limited to the remaining three, and so on for the first four players. If you are the fifth or sixth player to pass through College, you may not score any of the specialized degrees, but do score the regular College Degree and the normal salary increase of $20,000. Any College Degree, specialized or regular, may be used for entering Big Business. A player selecting the degree in medicine becomes the Doctor. He or she receives payments for players in the Hospital; the Doctor does not have to pay upon landing in the Hospital and may move out of this space on his or her next turn. (Note: If the Doctor is instructed to go to the Hospital he or she must go immediately even though they do not follow the instructions printed on the square when they get there.) No player may go to College more than once.
I'm not an expert at the game by any means, but I would infer from the sentences I've highlighted above that, in fact, there can only be one Doctor in any one game of Careers.