It is rather difficult to troubleshoot this scenario on a platform such as this one.
Unless you entered the transaction twice (for example, once in the Rentals section and again in the Sale of Business Property section) there appears to be no reasonable explanation for the gain to have doubled, absent some other entry or program error.
As a result, I suggest that you contact Support via the link below.
What is the TurboTax phone number? (intuit.com)
