Hard disagree for Echo - the enemy team isn't inclined to position themselves to make it easy for you to target your desired hero, and will typically be actively shooting you when you're trying to target Duplicate. Without confirm Duplicate, it's very easy to target the wrong hero.

For reactive Duplicates, the extra time from having to double tap Q rather than single tap it is fairly minor, especially compared to the startup time Duplicate itself has.

Since it also keeps the target for a few seconds, if they don't die, you can also use it to reposition slightly before actually starting Duplicate. For example, for Ball and Doomfist, you can position yourself above a group of the enemy team and immediately Piledriver / Seismic Slam to get a large amount of ult charge. (In Ball's case, it's feasible to fully charge Minefield with enough targets hit.) Those two will typically be flanking, rather than grouped up with their team, so a few seconds between targeting and ulting can be very helpful.