Duplicate stuff in honkai ​

Duplicate valk cards ​

• get converted into frags, which let your rank up the same valk
• if already sss-rank, additional frags can be used to buy witch orbs, useful for buying normal crystal cores

Duplicate stigmata ​

• gacha/bounty stigma can be converted into 1k red reson, useful for buying stigmata from exchange
• crappier ones can be converted into asterite or normal crystal cores

Duplicate weapons ​

• gacha/bounty weapon can be converted into 1k blue reson, useful for (1) upgrading pri-arms (as torus) and (2) buying weapons from exchange
• most other 4-star weapons can be converted into 1 honkai cube, useful for upgrading most 4-star weapons
• crappier weapons can be converted into cheap mats