A repayment made in 2019 will not reduce the taxable amount of Social Security benefits on your 2018 tax return. It will instead reduce the gross amount of your 2019 Social Security benefits, or if the repayment is more than the amount of your 2019 benefits by more than $3,000, you can use a claim of right on your 2019 tax return.
See Repayment of benefits in :https://www.irs.gov/pub/irs-pdf/p915.pdf
