As a native Canadian English speaker, I would use 'that' twice if I was emphasizing the second 'that': "Are you sure that THAT is mämmi?" Otherwise, I'd more likely say, "Are you sure that's mämmi?".
A native Irish English speaker agrees completely!
1st that = conjuntion
2nd that = pronoun (try replacing this with he/she)
Yes but the conjunction can be removed without changing the meaning. Indeed, it would be removed by the swathes of English speakers who choose to avoid saying "that that".
So "Are you sure that's mämmi?" should also be accepted.
As a native English speaker, it feels odd to have to say "that that" (even though it is technically correct) and I would normally only use one. "Are you sure that is mämmi?" feels much more natural.
why is "certain" not correct?
"You sure that's mammi?" is common.
"You sure that's mammi? Sure doesn't look like it to me."