I am a Chinese applicant for Churchill college of U.cam in the 2018-2019 UCAS application. I am applying for math and I have received the e-mail from the admission office overseas of Cambridge, informing me of the overseas interview at Oct.25th and 26th. Then today I receive another e-mail from Churchill college about the interview in Churchill from Nov.29th to Dec. 7th. I wonder what’s the difference for these two rounds of interviews.
Main difference is that you have to pay for the overseas interview / you don’t have to get a UK visitor visa 
The interview format & at-interview testing are the same.
You would have had to apply for the overseas interview, so the offer for an interview in the UK may be a mistake- you don’t do both.