The problem you're describing is that your current year capital gains (line 12700) are being completely offset by the same amount showing up as applied capital losses from other years (line 25300). This isn't necessarily an error - it might be TurboTax correctly applying your previous losses to reduce your taxable income.

Here's what's likely happening:

1. You have capital gains this year on line 12700
2. You have unused capital losses from 2018 and 2019 that TurboTax is automatically applying against your current gains on line 25300
3. The negative amounts you saw were likely your actual losses (which are negative by nature)
   
   

To fix this:

• Go to the Capital Gains/Losses section

• Check the "Capital Loss Carryover" screen

• Verify the amounts from 2018 and 2019 - these should be positive numbers representing your losses

• If you don't actually have these capital losses to apply, delete the amounts

If the issue persists, you might need to:

1. Go to the Forms view in TurboTax
2. Locate Schedule 3
3. Verify your capital gains are correctly calculated
4. Then check Form T1A to see how your capital losses are being applied

The system shouldn't duplicate your gains as losses unless you actually have losses to apply. If you can't resolve this, I'd recommend contacting TurboTax Support directly as they can review your specific return details.