Nashgar dupes were very popular before season 2. Sq. Wabbit was better but much more rare than him, so Nashgar got a lot of duplicate use. But with season 2, Namahage outclassed him. IMO Nashgar dupes are still good but only if you don’t want to wait for more Namahages.
Ulmer should not be duplicated, but one of him is good to have since he has the highest blue 3* tile damage. Valen is generally not considered as good as Gato, but since he decreases defense like Ulmer you can mix and match. Personally I go with 2 Valens, 2 Gatos, and 1 Ulmer.
You forgot Mnesseus in there He’s a bit better than Berden for damage although Berden’s atk- is nice. Hisan dupes are the best though. Belith is good for clearing the event but not very good for high scores. Depends on goals. Oh yeah… and Muggy, I think one of him would be worth for Avalon, his tile damage is nuts. He’s yet to be tested though but I have a feeling he’ll be a prime candidate for green 3* duplicates.
Agree with yellow, I see more duplicate Banes than Melias myself though, even though Melia has higher tile damage her skill isn’t as damage dealing as Banes.
I think Balthazar is the only one worth dupes for challenge events, strong snipe just can’t be beat. Gill-Ra again one of her is enough, she is good in challenge events for decreasing defense though. Chochin imo one is also enough, he helps more so in the later stages to help spread damage on enemies.