I doubt that it's even possible to avoid overwriting memories. The very nature of the way memory works makes that unlikely. People can train themselves to improve their memories, but there are still fundamental limits. There isn't even any proof that eidetic memory actually exists outside of fiction, and even if it does, it would be constrained by the sensory limitations of the observer. It would simply be a memory of what they had perceived which is as vivid as if they were still directly perceiving it -- but their actual perception, and their ability to interpret and understand what they were perceiving, would be no less fallible than anyone else's.
Heck, historians and historiographers won't even accept films or recordings as absolutely unbiased evidence, not without corroboration from other sources. After all, even raw film or video is a biased account due to the operator's choice of where to point the camera, what to emphasize and what to exclude from the frame. Every account is an interpretation, not an unfiltered truth.
