Don't sweat it, man. With your luck, that's not the last time you'll see him. Now you know how to handle him properly when you do get another copy, and you'll already have the first one ready to feed to him!
If it's any consolation, the two highest-level units in my entire roster after Lilia aren't even present in my main team. They're Hunter Stray and Commander Julian. Stray I leveled up early on when I was trying to get geared up for the Thurs. H daily. Game was still new to me, and I thought I could make up the difference of no affection vs max female affection on my silv archers by just power-leveling Stray. I hit level 50CC52 before I realized what a wasted effort this was.
Julian was much the same case. I got lucky with gatcha rolls and had multiple copies of him, and I knew he'd drop from certain events (like this one) so I figured a max-cr Julian could easily replace Katie. Unfortunately, Julian has arguably the worst line-up of stats out of the three gold rarity soldiers, and a not-so-super Awakening passive. I realized then that Katie was still the better investment of the two, especially when paired with Aria. Now that I've done more research, I feel like I could swap out Katie with Jerome, but Julian now sits like a gold-rarity, 50CC52 paperweight in my roster. Well, I also thought I would need him for the Mens Battle maps, but as it turns out I don't even need him for that either.