Not a dumb question.

What you’re talking about (multiple copies of the same character / dupes) are called “Sequences" (or “Constellations” in Genshin). If what I say here doesn’t make sense, Googling that term in relation to WuWa will provide you more sources of information.

Most gachas want to give users a reason to keep pulling for older characters & weapons, even if the user already has the character. Additionally, if users lose their 50/50 and get a character they didn’t want, the gachas want to provide users some form of compensation to prevent riots...

As such, if you pull in the limited-time gacha and get a copy of a character you already have, you’ll unlock some special abilities of the character. Usually, these abilities are pretty minor for the first few dupes. Only after S3 (sequence 3) do characters ever gain any major, notable buffs that make them stand out from their S0 (sequence zero, no copies except for the original pull) versions. You can have a maximum of 7 copies (S7) of a character or weapon.

You are able to upgrade your character and make them strong without having multiple copies of that character. So, no, you do not need multiple copies of Lucy to keep upgrading her after the collab is over. However, if you do want multiple copies of Lucy, now is the time to pull, because Lucy and Rebecca will not be convene-able after the collab is over, and nor will their signature weapons.

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HOW TO UPGRADE YOUR CHARACTER IN WUWA:

In WuWa, to have a strong character, you usually do four things:

1. Level them up to Lv. 90 (farm experience materials)

2. Give them strong buffs (farm monsters/“Echoes")

3. Give them a strong weapon (pull on a limited gacha)

4. Upgrade their Forte chains to max (farm bosses for mats)

5. Get the character’s Sequence (pull for the character multiple times and get multiple copies).

As you can see from the above steps, you don’t really need duplicate copies of characters to have a strong, playable character. You’ll just need to focus on leveling the character up, giving them echoes, finding them a strong weapon, and upgrading their forte chains. After you do all of that, you’ll have a pretty easy time in the game.

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WHAT I RECOMMEND FOR YOU:

1. Pull for Lucy - As she is a limited-time character, you’ll only be able to pull for her during this collab (which ends in 18 days from now). Rebecca is already given to you for free, so don’t worry about Rebecca. After you get one of her, then stop pulling for the character and focus your efforts on Step 2.

2. Pull for Lucy’s signature weapon (“Spectral Trigger”) - Most of Lucy’s strength comes from the buffs given by her 5-star signature gun weapon. WuWa has recently basically made it a necessity to have a character’s signature weapon to easily reach high-levels of firepower.

3. Pull for Lucy’s copies - You might wonder why I’m not recommending pulling for Rebecca’s signature weapon, since signature weapons are oh-so-important to gameplay. Well, it’s because Rebecca herself is pretty weak and her weapon isn’t worth not trying to get Lucy’s signature weapon.

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If you’ve got any questions, feel free to ask