There are two types of researching deja vu's. The first literally is, have I done this before? Have I already looked at this book, this microfilm? The other type is, have I forgotten something? Did I not look in that book?
Part of being a genealogical researcher that is nice, is the ADD nature of the work. After researching one branch for a while you drop it and start on another branch. If you've been working for a while you are probably working on several branches at the same time. If you're me, you've got all of the branches memorized so that you can research any branch at the drop of the hat. [Middle age has dulled my sharp edge a bit--so now I need my Iphone with a genealogical app on it for backup--but as of today, just backup]. Part of working on the Wallace family has brought these feelings up since this is probably the third "real" round I've gone with this family. I was panicked the other day to think that I hadn't looked up Isaac Wallis's death record in the statewide New Hampshire VRs, and only in a book. He was not in the statewide VRs.
In 1990 I started keeping a research notebook. This was an effort to cut down on the deja vu possibilities. Part of that first notebook is transcriptions of materials. I hated paying for copies and the digital era had yet to dawn. The problem is, I kept the notebook in chronological order of my own research. So it jumps around, family to family and geographic area to area. I still need to flip through it (usually from beginning to end) to find something or confirm something. For instance in 1993, I have the transcription of my Holic great-grandparents' passenger list from 1900. Under that I have the death record of Charles Luther Pinkham. Next comes the fax number of the town hall in Alma, New Brunswick, Canada. And so it goes through two large research notebooks.
Much of notebook #1 got entered in to my genealogy database. It was painstaking and tedious, but necessary. The most recent notebook starts in December 2004 so, it is just a listing of research items and whether or not they were completed. Still and all, for someone who tries to be very organized, I'm amazed at how unorganized my research always seems (to me). Thus, I'm always in the throes of deja vu.