Vanguard does not send it to Turbo Tax. You either imported the 1099B or manually entered it. You had to double check it before you filed.
Turbo Tax is a self prepared return. It has no way of knowing or checking that you entered something wrong or in the wrong place or entered something twice It doesn't check against any outside sources. The IRS has copies of all the W2s and 1099s your employer and banks/brokers, etc. sent in to match against. Turbo Tax only checks the calculations of what you entered. It is your responsibility to check it over before you file.
Well, since the tax return is "user" prepared, then you are responsible for making sure everything is shown in your tax return. It could either been something you forgot to check on, or possibly a "Beyond that" explained below:
Beyond that....folks who own REITs should always wait until mid-March to import their brokerage forms....Why? Because some REITs are allowed to report their data much later than other funds/stocks etc. A lot depends on the reporting behavior of your particular REIT(s). As such, your brokerage may have issued an initial 1099-DIV that did not contain the REIT information, and then a corrected 1099-DIV by ~1 March that did include that data.. As such you may have downloaded that 1099-DIV, and filed before your REIT data was included.
For Vanguard, I saw one 1099-DIV footnote re: their 2019 tax forms:
"*Real estate investment trusts (REITs) generally can’t calculate their taxable earnings until after
the end of the tax year. For this reason, we request an extension from the IRS each year to
mail completed Form 1099-DIVs for Vanguard Real Estate Index Fund."
And the 2020 "expected dates for 1099-DIVs is 19 January 2021...except for REITs, where 2/18/2021 is "expected", but not necessarily locked in for that date. I have no idea when they issued them for 2018.