Whenever the samall ‘っ’ comes up in a word, it wants me to write ‘tsu’. But it’s supposed to double the following consonant. こって isn’t written or pronounced kotsute, but kotte. How can I correct this issue in the japanese course?
Are you using a Japanese IME on Memrise? If not, I strongly recommend that you install and use one, so you can type directly in Japanese, rather than in romaji. Memrise has some issues with accepting romaji correctly, such as the one you mentioned, and it can quickly turn into quite a hassle. Using an IME helps a lot when you get into needing to type kanji, as well.
This is the article that introduced me to IMEs, so it should point you in the right direction:
Feel free to ask if you have any questions about it!