I'm sure the interviewee doesn't mind this because it makes him sound more eloquent. The reader probably doesn't mind either because for them clarity is all. But I think most journalists would consider this sharp practice, particularly if the quotes are not flagged up with something like "as he said in an earlier interview". He may well have said it but the fact is he didn't say it to you.
Interviewing is like fishing. Sometimes you get a bite. Most of the time you don't. In fact increasingly you're going to a lake that has been intensively fished for some time before you got there. Interviewees don't have an endless supply of original things to say. Mostly what you get is what they've been saying to the person who interviewed them half an hour ago. You may get a slight variation but the essence remains the same. All that makes your encounter distinct is your ability to write a more nuanced account.
The current pretence that each encounter is in some way exclusive is dear to the hearts of editors and journalists, who think of themselves as competing in the traditional fashion. The new way, in which all information and opinion merges into one giant Wiki, is the way of the future. And where do big-name columnists and the newspapers who pay them stand in that world?