Once you get good testimony at a deposition, you're best to leave the testimony alone. Take the good answer from the witness and move on.
It seems like obvious advice, but some lawyers can't wait to read the good answer in the printed transcript. Perhaps not believing their good fortune, or perhaps wanting to relive the excitement of the five minutes when everything was going their way, they return to the question again a little later in the deposition. They ask the question to which they got a good answer a second time.
Why give the witness a chance to change or explain his answer? I can think of only a couple of reasons, neither of them very good: (1) You're not sure whether you've pinned the witness down or (2) you can't remember what the witness said.
Both of these possibilities should be dealt with on the first go-round. First, pin the witness down after you get good testimony. Second, take good notes. Then move on to the next area of questioning--never to return until trial, when it will be much easier to impeach the witness than during the deposition when you have no transcript.