What to do with extra copies?

summer tikis keep the best one for viability and hero merit and use the other for inheritance i recommend to inherit the weapon since its unique

Brave Lucina just merge her skills can be found in units in the 4* pool
Regarding ivs thats difficult to say but she is a support so can go either way.. but i preffer +def -res her weapon even gives +def and with fury you get the speed increase alongside defense dont forget that she doesnt like getting doubled