Problem

One of my flights scheduled for yesterday is not showing up in Flight Watch when I look at yesterday. However it is doubled today?

Solution

Flight Watch works off revised (actual) times.
If a flight is delayed past midnight (into the next day) it will show in Flight Watch on the next day.
You are seeing 2 flights that day due to the one scheduled for that day and the one delayed into that day.
Flight scheduling works off scheduled time so will display it on the original date not the revised.