Wave 7 meta discussion was dominated by Twin-Laser Turrets, and how to counter them. The developers mentioned in an interview that playtesters had not tried to pack four of them in a list, and the results were a bit surprising. If my memory isn't faulty, they then went on to mention that crack shot was stronger than expected in multiples.
We have seen that effect with Crack Shot, a single one doesn't seem super useful but five of them on A-wings and six on TIE fighters has been a very strong list. The effect isn't as strong on Twin Laser Turret, I have seen singles on Miranda or a HWK, but they are much less scary when there is only one.
The conventional knowledge is that spamming a single ship or a single upgrade enhances both their strengths and weaknesses. They become more of a hard counter to something while getting harder countered by something different.
Getting a bit farther off the beaten track, I have found that Feedback Array is similar. A single one is okay, probably useful. Three of them is terrifying to PalpAces. Some people talked about running seven of them in the Bugzapper Swarm, though I've never seen it in person. When it was talked about, you would have needed to buy four Aggressors. Now, however, brobots and triple jumpmasters should give all dedicated scum players seven copies.
I think munitions could also count as something that gets better in multiples. A single torpedo isn't that bad to set up facing, but a list that will probably shoot three (or more!) at the same time is something that you need to think about in list building now.
What do you find to be better the more of them you have? I have a tickle in the back of my mind that four to five Dead Man's Switches might make them better than anyone has thought to this point. Maybe with something that can drop a proton bomb...