When running day end the last couple of days we get the following message: " Document 0 18454 already exists in history data file. Do you wish to overwrite? What does this mean and why is it all of a sudden coming up? I don't want to overwrite without knowing what it is but it does not give an option of changing the name or file.

Order #18454 already exists in history when it goes to save the order. Open up history and look at the already saved order. Decide which one you want to keep and delete the other.

Usually this happens when an order is completed (and copied to history), and after that the order is edited and changed.