One of the things that I absolutely hate when I am researching is to pull a resource - a book, a microfilm or anything else to search for a specific item and find that I have already looked for that item in that resource before. This is particulary true for newspapers.
In order not to waste my time searching a second time for items this is what I have started doing. Instead of deleting the to-do item off my to-do list I simply mark it closed, leave it on the list and make notes as to what papers I have searched and the dates that I covered.
The next time I go to add that item to my to-do list I can clearly see that the item has been attempted and I move on to either a new resource to check or go on to other things.
Sure saves on the duplication and gives me time to search for new things!