If I enter the property tax paid in the two areas where it asks for it, Turbo Tax adds them together. Am I misreading something? Under the personal deductions and credit section it says "You reported $xx.xx in property tax on your home office. This amount should be the total property taxes paid on your main residence." That sentence is not clear if I am supposed to enter the same dollar amount or enter zero since I already entered it under the Business Income & Expense section.
Enter the total amount of property tax paid in both places (business expenses and personal deductions).
In my test case, I first entered $20,000 in property tax in the business expenses. I also entered home square footage as 3,000 and office square footage as 300, so the amount of home expenses like property tax to be allocated to the business is 10%.
When I went to personal expenses for Schedule A, the field for Property Taxes was already filled in with $20,000. If so, just leave it. If it's blank, then enter the same amount as you entered back on the business expenses page (i.e. the whole amount, like $20,000 in my example).
When I had reached a stopping point, the amount of the property tax allocated to the business was $2,000 (10% of $20,000), and the amount of property tax allocated to Schedule A was $18,000, just as you would have expected.
In other words, put the total amount paid in both places. If you set up the allocation process correctly in the business section, TurboTax will correctly divide the amount between your business and your itemized deductions.
I am a bit concerned that you said "It ends up doubling" - go back and re-enter the numbers the way I said, and then check the output (this means that you will have to manually compute what your Itemized Deductions should be and what your net Schedule C income should be, for comparison). If you are doing this online, you can see the results on the left at Tax Tools->Tools->View Tax Summary.
If you are doing this with the desktop product, I believe you can see the actual forms (Schedule A and Schedule C) by going to Forms.
So far, it looks like online and desktop look to be operating the same way, although I understand your point about possible confusion on what to enter. Even when I deleted TurboTax's $20,000 entry and coded $20,000 again (like you did), the calculations remained correct.
It seems you are on the ball and have done your own due diligence to verify that the deductions are correct and in the correct places. Since we on AnswerXchange can't see your screen, if you want to pursue this, I would suggest that you contact phone support where an agent (with your permission) can see your screen.
The bug is still there in 2021.
On the "Enter any additional property tax you paid in 2021" page. It has the entry that I previously entered.
But if I leave the Property Taxes box blank. It will double my property taxes deduction minus the home office portion.
I have to enter "0" in the field in order for it to calculate it correctly without doubling the property tax deduction amount.