Sometimes a person needs to repeat Shmoneh Esrei. Some example cases: tashlumin, if one forgot ya'aleh v'yavoh, or said the wrong birkat hashanim.
This person is davening Shacharit, and immediately after he finishes his Shmoneh Esrei, he realizes that he needs to repeat it. Should he start right away, or say (at least the beginning) in sync with the chazzan? Does it make a difference why he's repeating it (mistake in this one, or tashlumin)?
If he says it with the chazzan, can/should he say kedusha also?