It's a double standard if they still allow stacking of other missions, like data delivery.
That's a different type of stacking entirely.
If you take 20 data delivery missions, you have to hand in 20 data delivery missions
If you took 20 massacre/skimmer missions, previously you only had to fulfil the requirement for ONE mission to complete them ALL
Now, if you take 20 massacre/skimmer missions, you have to complete 20 massacre/skimmer missions.
It's the exact opposite of a double standard. It's standardising the way missions work. It's bringing the massacre/skimmer (and settlement scan) missions in line with all the other missions in the game so that you have to complete each mission individually rather than taking a bunch and rendering the requirements for the others redundant.
How is it logical that 2 people want the same target dead, but after I kill the target person #2 says "well hey, you didn't kill it for me so go kill another one".
Zero logic.
That's not how it works either. You're not given a discrete target, it's a massacre. If it were 1 faction has 10 missions to kill the same pirate, then yes, once should be enough for all. But the question then would by why would 1 faction give out multiple contracts to kill the same guy?
But no, how it works is 1 faction gives out multiple contracts for kills of a
people group. In this case it makes sense for taking multiple contracts to add up. The mission giver is then saying "So you've taken 3 contracts at 30 kills each, so you're going to kill 90 ships for us? Brilliant, that'll show them! Remember, I'm paying you for every 30 kills you make."
You're missing the difference between two different types of mission. One is specific, the other is general.