This occurs on almost every popular SE site. I recommend migrating this to Meta SE.
Assuming this issue may affect the meaning of reputation, or cause true disturbance in the system, is quite a stretch.
Boosting friends is bad.
However...
Disclaimer: My experience comes from Stack Overflow, where this behavior is very common.
Addressing some of your statements would help express why this is a non-issue in terms of bringing harm.
I found your post by searching for this same issue, but for SO.
Reputation is supposed to reflect the technical quality of users' answers over time
#Reputation doesn't express current ability
Ranked #17 on Stack Overflow for all-time reputation. Hasn't contributed a question or answer since July 12, over 5 years ago in 2012.
His content is still useful. The quality of his posts seem up to par. It fits the philosophy of SE.
But, for all we know, this user could have switched hobbies/careers since, got rusty, and they may struggle contributing properly in modern times.
This proves reputation isn't based on current capabilities.
Adding the issue of answering blantant duplicates, reputation appears to signify the amount of traffic you can generate.
Our problem with this sort of behaviour is — to put it bluntly — that it undermines the whole system
#The system undermines itself
10 upvotes. 20 downvotes. 60 rep was gained.
Users' reputation would mean nothing if SE died out. So why risk inflation of duplicate content to gain rep?
I can't easily advertise maintenance. Yet, marking duplicates properly requires you to not only understand the question, but understand the site.
Those who help maintain the site by linking related information through duplicates get rewarded nothing.
But write an answer, have the OP upvote & accept, and you can gain 25 rep (10 for vote + 15 for accepted). Downvoted 10 times? -2 * 10. Still made gains.
reputation now also reflects the number of friends a user has
#Not even close
These actions are obvious when performed in a way that would cause any harmful or noticable impact.
If reputation started reflecting popularity, it wouldn't be the fault of the users, rather the system itself. Automate it.
The system has no issue automatically banning someone for removing content they contributed. Yet, it's up to the users to ensure the site remains clean?
Friend boosting becoming an impactful issue on the exchange sounds like a joke. SE has already proven it's capabilities to automate tasks. So why not this?
Should we also ban ourselves for deleting our own content?
#Your sugestions
In terms of what users can do, I have no suggestions. Rather, this section of the post explains why your suggestions wont work.
In previous meta posts, I touched on the issue of rep hounding. There are reputable members doing things they know goes against the philosophy of the site.
For newer users who aren't familiar with how the site works, asking or answering a blantant duplicate doesn't seem malicious. But friend boosting? That's not a given?
If new people don't care to check out the tour or help center when asked... If reputable users don't care to maintain the site's philosophy...
What makes you feel anyone would follow this? What do they gain?
#My suggestions
1.8k rep. Across 3 users, a combined total of 1.8k rep.
Would anyone care to go out if their way to prevent such small impact on the system?
"Stop it before it becomes an issue" is what one would say if they were oblivious to the much, MUCH, larger issues, which have a far bigger impact on the exchange than 1.8k across 3 accounts.
If you fear of it expanding, dish out some automated temporary suspensions.
Do this too much and you'll find out what losing a thousand reputation in a second feels like.
Who is your target audience here? Regular SE users, or people who may contribute for about a year, then leave?
If I lost 1k rep, out of my small total of 8k rep, I couldn't say I'd care. Reputation has lost it's meaning to me.
But for those who do care for rep... 10 upvotes, 20 downvotes, +60 rep. Come on..
I love the site, which is why I'm somewhat heartbroken that it has come down to this.
#It's as if the serious issues won't be addressed properly, so we have to make 1.8k seem like a big deal.