Situation 1:
On obtaining a 5-star character that you already own (whether obtained in a wish, redeemed at the shop, or awarded by the game): The 2nd – 7th time you obtain the character, it will be converted into that character's Stella Fortuna ×1 and Masterless Starglitter ×10; from the 8th time onwards it will be converted into Masterless Stella Fortuna ×1 and Masterless Starglitter ×25.
Situation 2:
On obtaining a 4-star character that you already own (whether obtained in a wish, redeemed at the shop, or awarded by the game): The 2nd – 7th time you obtain the character, it will be converted into that character's Stella Fortuna ×1 and Masterless Starglitter ×2; from the 8th time onwards it will be converted into Masterless Starglitter ×5.
You can refer to [Wish] -[Details] in-game, and can also get more details from the official post: https://genshin.hoyoverse.com/en/news/detail/159082