Because you don’t need to copy an Echo as Echo to get Echo’s ability.
All you’ve gained is faster ult charge which you wasted a completely charged ult to obtain in the first place.
If you copy a non-Echo hero after copying Echo first, you get less time on the non-Echo hero than you would have if you just copied that non-Echo hero in the first place…reason being you spent a few seconds charging another ult, which ate into your duplicate duration.