Staring at a page and repeat
When you start at a blank or partially complete slide, it is tempting to put that key message in. You don't have the overview of the entire presentation, so the key point will appear - duplicated - on many pages.
The same thing I often see happening on corporate web sites: multiple people make edits on different pages independent from each other.
The slide and the page on its own make sense, the overall story looks garbled and duplicated.