i want to say that there is one JP mobage game i played that you do need same character to limit break. it is tales of link (i played global version). the difference is you dont need to evolve it all the way to 5* like in VF since the gacha is rarity based. and also there's few event that give a character that can be used to limit break gacha character. I'm still fine with the way of limit breaking, but pretty much i hope they could really make it easier so that we dont need to evolve said same character to max level (job key is limited, if only its unlimited then by all mean i will support same rarity/same class limit break)
I too have played Tales of Link, and I think their Limit break (LB) system is quite nice. For those who haven't it basically works like below:
1. Characters are fixed at 1-5 Star and have varying max level caps based on star rating.
2. To get a 5 star character to max level you would need 5 copies of a character (Base + 4)
3. While you can use exact copies of a character to LB, they also have these characters called 'Hawks'. Hawks basically act like a duplicate of any card in LB, if its the right type (bashhawk for bash characters etc.,). Essentially you can have 1 gacha character, but then use event hawks to LB them.
4. Hawks have only be recently introduced to Global, so we can't tell how common they will be. But I would be surprised if they didn't show up in most major events (every 2-3 months).
Disclaimer: I haven't played Valiant Force so I'm not offering judgement or claims on VF. But as people are looking at Limit break systems I thought it would be nice to see another similar system for ideas.