When I'm driving, I often want to know the time. I can look above the steering wheel, to see a digital clock, or at the navigation system, to see another digital clock. Unfortunately, these times are almost never the same! They disagree by a minute or more. So, what time is it?
The software development life cycle (SDLC) gets very complicated and involves many engineer-hours of work, plus other investments. There's a lot of information involved.
Yesterday I attended an excellent seminar by Jim Benson (personal blog here http://ourfounder.typepad.com/ ) about Personal Kanban (Jim's book of that title is e.g.here http://www.amazon.com/Personal-Kanban-Mapping-Work-Navigating/dp/1453802266/ref=as_li_ss_mfw) and it was very enjoyable but I was bothered by the reliance on sticky notes for designing and communicating informtion. Those sticky notes show up across all sorts of agile methodologies. I can see the advantages: it's very social to be collaborative with your team in a room with the sticky notes, and the muscle memory and physicality of moving the stickies around helps communicate and absorb information.
But, I asked (OK, pestered, but Jim was a good sport about it) several questions along these lines: do we really need the sticky notes? There's cost and risk in relying on people to manually translate information from plans or some virtual document to the stickies on the board, then after the meeting with the stickies or at some other time (depending on how the team does it) this has to be carried over to the IT workspaces or documents. There are many potential points of failure, and many potential points of information duplication.
The problem with duplication of information is the same with the two clocks in my car: they can easily get out of sync, and then which do you believe? Information can get lost too, if I reorganize the stickies thinking that someone has already done the maintenance step of writing the information to the appropriate document, where actually that hasn't happened for some reason.
I predict that in a few years, the stickies will be gone because there will be a sufficient hardware and software solution to solve all of the problems that the stickies do, without the cost and risk. Team communication around work items will be more robust and efficient. There won't be any stickies to flutter to the floor (as a few did, during Jim's talk).
Worse than the clocks in my car, and even worse than the stickies, is superfluous docs that get out of sync. By "superfluous" I mean people ignore them, so they get out of sync and/or out of date, so they can cause confusion; for example, a doc that lists test cases that also are in a database.The test cases are probably going to change, and there's a very good chance that the doc will get out of sync, and there's also a good chance that someone will rely on the doc when it represents incorrect information.
Better to limit a document to information that doesn't exist elsewhere, and link docs to other docs and resources (databases, source code repositories) so it's clear where to get and where to update information.
Even worse than all of the above: duplicated logic in code, or duplicated code. See posts here http://metaautomation.blogspot.com/2011/09/beware-duplicated-code.html and here http://metaautomation.blogspot.com/2011/09/test-code-quality-readability-and.html .
Use team best practices when writing, and don't be afraid to clean up unused code! Duplicated logic can haunt the team and the product quality.
There are times when information duplication is by design and there's a robust system to keep it so, e.g. databases that are de-normalized or replicated or diagrams to visually represent a coded system... OK, so long as the diagrams are frequently used and updated by stakeholders on the team.
Beware the hazard of things getting out of sync. If the clocks disagree, they both look wrong!