Passion’s Flow:
If unit initiates combat and is adjacent to ally who shares A rank support, attack twice (brave).
Double Team:
If unit is adjacent to ally who shares B rank support or higher, both units can double when initiating.
Cautious Plan:
If allies within 2 spaces share C rank support or higher with unit, unit and allies can’t be doubled when attacked.
Blame FEH
Might as well continue the soul skill line.
Pegasus Soul: If the unit doubles, move again
Pegasus Soul+:If the unit doubles, move again (+5 Move)
Falco Soul: If the unit can double, can attack again (+5 Move)
Triangle Block: If unit has WTA, enemy follow up attacks deal half damage.
Triangle Flurry: If unit has WTA, follow up attacks deal 1.5x damage.
Try Again: Unit’s follow up attacks are always guaranteed to hit.
Perseverance: Unit’s follow up attacks are always guaranteed to crit.
Departing Gift: Unit recovers HP equal to damage after a follow up attack.
Demi Luna: Unit’s follow up attacks always avoid defensive stats.
Sureblaze Adept: Unit performs a third follow up attack at +8 AS over enemy.
Surefire Adept: Unit performs a third follow up attack at 0.5 damage at +6 AS over enemy.
Swift Feller: Unit’s follow up attacks ignore enemy’s terrain bonuses.
Fireflurry: Unit’s active offensive support bonuses are doubled on follow up attacks.
Iceguard: Unit’s active defensive support bonuses are doubled against follow up attacks.
Fireguard: Unit’s active offensive support bonuses are doubled when the enemy can follow up.
The names are whatever.
I listed some vanilla preexisting skills from last time Vesly asked for buff skills, so here we go again.
Bold Fighter: If unit initates combat, unit doubles.
Vengeful Fighter (A weaker Quick Riposte): If defending and HP is 50% or higher, unit doubles.
Brash Assault: If unit initiates combat against a foe that can counter and units HP is 50% or lower, unit doubles. (This is similar to AdVantaged and Shuusuke’s “Confident Strike”)
Odd Follow Up: If it is an odd-numbered turn, unit doubles and foe cannot double.
Even Follow Up: If it is an even-numbered turn, unit doubles and foe cannot double.
Dive Bomb: If both combatants are at least at 80% HP, unit can double before foe counterattacks (like Desperation - this isn’t a free double)
Deflect Melee: Reduces damage from melee follow up attacks, including brave weapons, by 80%.
Deflect Missile: Reduces damage from ranged follow up attacks, including brave weapons, by 80%.
Deflect Magic: Reduces damage from magic follow up attacks, including brave weapons, by 80%.
Binding Shield: Against dragons, unit always doubles. If initiating, dragons cannot counterattack. If defending, dragons can’t double.
Counter Slice: If a sword unit is attacked from Range and has 5 more speed than enemy, counter the attack and double totally not influenced Berwick
Evasive flow: On evasion, double attack.
Dangerous blow: Axe only. On initiation, enemy attacks first, but the user doubles after.
Forest Hunter: Archer/Sniper only. On Forest terrain, double before the foe counterattacks
Forest cutter: Axe only. When attacking a unit on forest terrain, double before foe counterattacks
Spearthrough: Lance only. When attacking a Sword unit adjacent to another unit, double and disable counter attack. Adjacent enemy unit takes 50% damage
Finisher: If foe’s HP < 50%, unit doubles.
Biding Blow: If foe cannot counter, unit doubles.
Follow Trace: If foe is under an status effect, unit doubles.
Ephemeral Reap: If foe’s total debuffs surpass a certain amount, unit doubles.
Impact Attack: If unit initiates combat, foe cannot double.
Defeatist: If HP < 50%, unit cannot double.
Slow Start: Unit cannot double during the first 5 turns.
I turned this into:
// Prideful Warrior: This unit always attacks after the enemy, but always doubles.
I’ve also made these two. Will need icons for these three if anyone wants to make some. Thanks!
Reckless Fighter: 
Keen Fighter: 
Prideful Warrior: 
One I’ve had mulling around was based off Double Lion if this counts.
Chipped Fangs: Half Damage but apply a brave effect.
Guardbreaker: if unit doubles, inflict -5 def/res depending on damage delt.
Relentless Drain: if unit has 5/more speed over opponent attacks gain lifesteal.
Battle tempo: if unit repeatedly doubles in combat, gain a stacking +1 to speed and defense.
? Ambusher: When under attack, this unit doubles.
Quick Learner: When attacking a higher leveled enemy, this unit doubles.
Steady Brawler: Deal 25% less damage when doubling, but deal 25% more when not.
If gaining support bonuses, this unit always doubles while attacking.
I’ve half made these 5 and will need icons. Thanks!
Bold Fighter: 
Vengeful Fighter Ambusher: 
Passion’s Flow: 
Quick Learner: 
Vengeful Fighter sounds better, thanks. And these all look great 
Steady Brawler:
Late, but still wanted to do one for Passion’s Flow
Will need an icon for this, too.
Bold Fighter
I made an icon for the skill I suggested. It’s an edit of the existing Wary Fighter. I made it days ago but never got a chance to upload it.
You probably want to add Null Follow-up skill that nullifies all skills that garentees follow up or prevents follow-up attacks. Also are you going to edit Quick riposte?
Wary Fighter exists
I will make one where you cannot be doubled
Quick Riposte will work the same as before
I guess vengeful fighter acts a Heroes quick riposte but Wary fighter is different than null follow-up. Wary fighter disables doubling for both units, NFU disables that disabling AND also disables skills that cause you to double when you otherwise would not. In fact it would just turn off all of these skills entirely and sets doubling back to if you reach the AS threshold to do so. I think its worth adding simply as an answer to these skills. In Heroes NFU shenanigans get a little complex but gba doesn’t have as much going on so it should be fine. Design wise it makes sense to me to have a way around having an entire stat ignored but thats just my opinion.
Made this, too. Icon please 
I’m happy to use yours over Someone’s since they’ve already made so many and you suggested the skill.
Likewise I’ve put your version into the pull request.
With this, I think I’ve made around 9 skills related to doubling and I’m done with them for now. Thanks for all the ideas, everyone!
I probably wasn’t clear enough in my first post, but the criteria I was generally looking for was:
If I didn’t choose your ideas, it was usually for one of the above reasons. Cheers!