What are the rules for taxation of foreign bank interest repatriated to US? (the principal was taxed already before it was invested in the foreign bank)

US Citizens and US Residents are taxed on their worldwide income, so it doesn't matter where the bank account is for income tax purposes.

Tax is based on income. Interest is income. You must report income in the year received. 

Transferring money between your own accounts is not taxable. It is not income. Doesn't matter if it is from one US bank to another or internationally. The IRS won't receive notice of it, though there might be some tracking for money-laundering/terrorist issues.

US bank will give you (and the IRS) a 1099-INT to assist your reporting (and let the IRS verify). Some foreign banks may do the same (US pressure if they have US operations). 

But even if you don't receive a 1099-INT you must still report the income. TT will ask you if you received interest. Just say yes and enter the amount for the tax year. 

You have to convert to USD. If you received multiple payments using the average yearly exchange rate is easiest. https://www.irs.gov/individuals/international-taxpayers/yearly-average-currency-exchange-rates

So basically don't worry about transferring between accounts. That is not subject to income tax. Report interest in the year received whether or not you get a 1099.

Note also you may be subject to reporting info about the accounts depending upon the amount of money in them. See https://www.irs.gov/businesses/small-businesses-self-employed/report-of-foreign-bank-and-financial-a...