Stimulus 2021 double payed for dependent that's no longer dependent

Each person's stimulus payment and amount are determined independently according to that taxpayer's tax situation and the rules in the law. Different people's stimulus amounts are not connected. From what I can tell, the stimulus amounts received by both of you are correct according to your tax situations and the law, and therefore nothing needs to be, or should be, "returned".

We can look at the text of the law for the third stimulus payment. (The conclusions will be similar for the first two stimulus payments, as the relevant text is similar.) The text of the American Rescue Plan Act is here. Section 9601 adds 26 USC 6428B which provides the third stimulus tax credit and advance refund. Subsections (a-e) provide for a tax credit for the 2021 tax year, and sets out eligibility for the tax credit based on various factors, including who are eligible individuals, income limits, money for dependents, SSN requirement, etc., from the 2021 tax year.

Subsection (g) provides for the "advance refunds" (the "check" or debit card) to be sent to people in early 2021. Subsections (g)(1) and (g)(2)(A) provide that the eligibility and amount of the payment would be based on the eligibility and amount of payment of the tax credit (described in subsections (a-e)) if it had applied to the 2019 tax year:

(g) Advance refunds and credits.—

(1) IN GENERAL.—Subject to paragraphs (5) and (6), each individual who was an eligible individual for such individual’s first taxable year beginning in 2019 shall be treated as having made a payment against the tax imposed by chapter 1 for such taxable year in an amount equal to the advance refund amount for such taxable year.

(2) ADVANCE REFUND AMOUNT.—

(A) IN GENERAL.—For purposes of paragraph (1), the advance refund amount is the amount that would have been allowed as a credit under this section for such taxable year if this section (other than subsection (f) and this subsection) had applied to such taxable year.

Subsection (g)(5)(A) substitutes 2020 for 2019 for people who have already filed their 2020 tax returns by the initial payment determination date:

(A) APPLICATION TO 2020 RETURNS FILED AT TIME OF INITIAL DETERMINATION.—If, at the time of any determination made pursuant to paragraph (3), the individual referred to in paragraph (1) has filed a return of tax for the individual’s first taxable year beginning in 2020, paragraph (1) shall be applied with respect to such individual by substituting ‘2020’ for ‘2019’.

Since you have not filed your 2020 tax return by the initial payment determination date, the "rightful amount" of advance payment you should receive, according to the law, is the amount of the tax credit if it had applied to your 2019 tax year, and that amount is $4,200, because you jointly filed in your 2019 tax year, and claimed one dependent in your 2019 tax year.

And since your daughter had filed her 2020 tax return by the initial payment determination date, the "rightful amount" of advance payment she should receive, according to the law, is the amount of the tax credit if it had applied to her 2020 tax year, and that amount is $1,400, because she filed as single and could not be claimed by anyone as a dependent in the 2020 tax year. (And in fact, even if she had not filed her 2020 tax return by the initial payment determination date, and thus received no stimulus payment initially since she could be claimed as a dependent for 2019, subsection (g)(5)(B) provides her an additional payment if she files her 2020 tax return by the "additional payment determination date" (90 days after the 2020 tax filing deadline), and the tax credit as applied to her 2020 tax year is more than she received -- which it is -- and she would receive $1,400 at that time.)

Nothing in the law provides that someone who received an additional amount for claiming a dependent will reduce the payment to the dependent (whose payment was determined under a different tax year), or vice versa. Nothing in the law provides that two people who claimed the same dependent in different tax years cannot both receive the additional amount for the dependent, if their payment happened to be determined based on the respective years that they claimed the dependent. Because people sometimes informally say that they "got the stimulus payment" of their dependent, you may have gotten a misconception that each person has (at most) one fixed amount of stimulus money that is gotten by only one person or another, but that's not how the payment amounts are constructed in the text of the law. In the law, the payment amount for a particular taxpayer are determined solely by that taxpayer's situation, with no reference to the payment amounts of another taxpayer.

There is nothing erroneous about either of those two amounts, and in fact, I don't even think that the IRS has the discretionary power to change the amount of the payment just because they think "someone already got the stimulus for this person", because how the amount is determined is set in the law.