Righto. Do not include on Schedule A line 5b the portion of real estate tax you deducted as a letting expense.
IRS Publication 527 says:
Be sure to report only the personal portion of your real estate taxes on line 5b of Schedule A. The personal portion of real estate taxes on the dwelling unit doesn't include the rental portion you reported on line 2b of this Worksheet 5-1 or any portion that you deducted on other forms, such as Schedule C or F.
Publication 527, Residential Rental Property
