In many of the resumes that I see, job seekers will often consolidate or "collapse" multiple positions at a particular employer into one position, often by simply noting the last position they held. The disadvantage of this approach is that it prevents potential employers from fully understanding the breadth and depth of your experience and as a result compromises the level of value that you are able to communicate. For example, if you held four positions at Company X over the course of eight years and during this time you earned three promotions, a potential employer is likely not going to be able to clearly understand or fully appreciate the extent of these accomplishments if your first three positions are omitted and only the last one is shown.
While it is sometimes effective to consolidate information about different positions for the same employer, especially earlier in your career (and sometimes it is necessary in managing a document’s length), it is generally a good idea to detail more experience as opposed to less so that you can give potential employers a fuller picture of your story and overall value, most notably the key outcomes that you achieved, including promotions, as well as the key responsibilities that you had. As a related point, this is the advantage of a chronological resume – and why many employers prefer this format – as it allows them to more easily and fully understand job seekers’ career paths and the full extent of their accomplishments and responsibilities.