Just a got a perfect level up (+2 +1 everything else) then consecutively gained +2 HP twice. :0 This has never happened to me on my previous runs lol. Guess I'm lucky :0 This was on Nowi btw. High HP growth?
Any growth above 100% CAN proc twice - Nowi as a Manakete has a 120% HP growth, so you basically have a 100% chance of +1 HP, and then another 5% chance for +1 HP (separate).
Donnel is probably the best example of this due to Aptitude - he can hit a whopping 120% HP growth as a Bow Knight due to Aptitude. I believe that Morgan can also break 100% HP growth as the daughter of a HP avatar+Nowi pairing; Nah will break 100% as well due to her mother.
Pr. Marth (DLC) is the only character I'm aware of that can break 100% growths in any stat other than HP; he does so with Luck (105%) alongside HP (in most but not all of his classes).
EDIT'd because I'm a derp and got Nowi's HP growth wrong somehow.
Another day, another unsuccessful genetic abomination!