If you made the $8,000 contribution in 2025, and directed it to be considered a 2024 contribution, then you will have an $8,000 basis reported on the 2024 8606. You won't be taxed twice, because when you report the conversion in 2025, you will have a nondeductible IRA's basis of $8,000 carried forward to 2025. You'll only pay tax on the conversion of the traditional IRA in the amount of $1,000 that you did not pay tax on yet.
Here are the full instructions to report a Backdoor Roth: How do I enter a backdoor Roth IRA conversion?
