While Devs did add continuing leveling at max tier, max level, you lose all the XP, so I tend to only use duplicates for the +25% skill roll.
If my 3* is stuck at 3/8, I can just level another copy for Alliance Wars, or so I have heard.
MY PERSONAL OPINION
I don’t know why special skills don’t automatically become 8/8 when you max out a hero.
If it is a program thing, maybe make all +skill rolls times the tier.
Example:
Tier of Leveling Hero, +% skill roll for 1* Hero of matching color, +% skill roll of duplicate hero (because Alliance wars)
1, +2%, +25%
2, +4%, +50%
3, +6%, +75%
4, +8%, +100%
Maxed leveling hero, +100%