They won't always recruit a 2nd time, as I've seen more and more sim teams with 3-4 walk-ons.
From what I have seen, they recruit a second time when human coaches have beaten them out for enough of the players they were already on that they need to do it to fill their quota of slots without walkons.
What that means is if a sim is "scheduled" to recruit three guys for four or five openings and is therefore already "on" three guys at the start of recruiting, and human coaches take two of those three guys from the sim, it will go after a couple of new guys.
What are the thresholds for when it will go after new guys? If you take one of four guys it went after for six openings, does it happen? What about if two guys are gone to humans? If the sim has two openings and one guy it wants and you take the guy, does it give up or go for another? I don't know the specific answers to all these questions or to all scenarios, but humans beating out sims for recruits seems to be the REASON why a sim "re-recruits".