Mark Hughes only played for us once. Not arsed about other clubs.
We've had the following have two stints, of which all bar none had the better stint first time:
Kendall
Dunc
Rooney
Gana (so far)
Unsworth (close call, but first stint was vital cog in successful Cup campaign).
Gravesen
McFadden
Beagrie
Jeffers
Donovan
Jo
Pienaar (close-ish, but most will agree first stint was a tad better).
Doesn't matter. It's still a simplistic sweeping statement. It's akin to the shouts of 'never buy a relegated player', a theory that coincidentally, Gana rips to shreds.
Just because that's historically been the case, is irrelevant. Every deal should be judged on a case by case basis. I'm not a fan of these generalisations.
John Stones would be better if we brought him back now. Bring him back in 4 years, and he probably won't be.
It's not set in stone that if we ever bring a player back, then they won't be as good. It might historically be the case, but that is based on nothing other than timing and the nature of the deals. It's not some unwritten rule that players will defo be worse if they come back for a second spell.