By pass...only once?

Hi all, thought someone here might know more about this. My father had a coronary artery bypass 15 years ago after suffering 2 MIs at the age of 47. He is not the type to ask many question when it comes to doctors. He says he was told at the time that the bypass could only be done once and if the replaced artery were to block again he could not have the procedure redone. He says it was the main artery and the bypass was taken from the mammary gland.
He has an echo and stress test every second year but, because he never asks questions, he doesn't know what that revealed. He gets chest pain when walking up hill.  He tells me that he failed his last stress test and was told to stop due to ischemic changes on the ECG plus chest pain. They didn't do any further tests.
So I am wondering if this artery is blocking up again and if it just a case of waiting around until he has another MI and then being able to do nothing. Is it true that another bypass cannot be done if the need should arise? Like I said he isn't one to question doctors much and I'm not sure if he's got his facts straight about his situation.
Thanks in advance to anyone who can help.

[deedle1976]

Just to clarify as my previous post doesn't read so well. My question is "can a coronary artery bypass be done only once?"