Someone anal explained to me once how you could use all the gears, and that you could print out the gear charts to tape to your stem so that you could know how to go through them on the right order... from first to second, to third to fourth, then shift to the big ring on the front while shifting down twice on the rear, then up on the rear while shifting down on the front...
It was really quite complex...
I just ride.
By the way, you can typically use all of the gears. Some gears are just not optimal.
Also gears are rarely duplicated exactly, they are just sometimes VERY close to one another...
For example a 50x36 compact double with a 12-25 10 speed cassette has no duplicate gears. For 50x34 or 52x39 rings with the same cassette each duplicates once... admittedly there are some ratios that are very close to one another...
So NOW who is anal???