Can a deposition in one case be used in a different case if the cases are similar and the witness is the same.

Can it be used? Probably. Possibly. First, there is an exception to the hearsay rule that allows former testimony to be admitted, but it is not automatic. The declarant must have had the opportunity to develop the testimony through cross examination and, really, the cases have to be substantially similar. Also, the court will not allow it if the court deems the deposition to be irrelevant or likely to cause confusion or unfair prejudice. Second, it could possibly be used to impeach the testifying witness. I don't know if this witness is available for trial or not in whatever cases you may be referring to (ongoing, future?). Have an attorney review your case -- if you don't already have one.