PASTEL wrote:

Mr. Smith is a teacher, who is also a doctor. Mr. Smoth is a teacher, who has taught Spanish for a year. Are the two sentences correct?

I wouldn't say they are perfect. It should be one of these:

• Mr. Smith is a teacher who is also a doctor. (I would use this maybe to explain a character in a movie.)
• Mr. Smith is a teacher and [he is] also a doctor.
• Mr. Smith is a teacher who has taught Spanish for a year.

Comma is not precise.

• Mr. Smith, who has taught Spanish for a year, is a teacher. possible
• Mr. Smith, who is also a doctor, is a teacher. ugly
• Mr. Smith, who is a doctor, is also a teacher. possible but not the same