Woah! Andrew and Katrina totally threw me for a moment.... my other half and sister-in-law's names... and their surname has a double R in it! Took me a second \;
I think this is a specific interaction between this set of quizzes and the word Ferrari. AFAICT all quizzes in this series require the double letter even where other quizzes would allow a single for a type-in, but since a reasonable taker could think it's 'ferrari' or 'ferarri' this quiz accepts both where it otherwise wouldn't.