i'm a korean.
-뜹니다. is not formal expression.
If someone want to show cuteness, the word goes like that.
like 'ㅃ', 'ㄸ', 'ㅉ' pronunciation, These consonant sound 'cute' to korean.
So they use these expression frequently with their friends.
(The Purpose of using these expression is so various.
With good manner, wife use these wording her husband with love.
Or Ridicule someone, or just kidding ...)
There are so many examples.
1) '그랬어요?' -> '그랬쬬요?', '구랬쬬요?' (You do~?)
2) '알겠음' -> '알겠뜸' (I see)
3) '~할거야' -> '~할꾸얌' (I will do~)
Additionally...This case is one of the reasons why korean language is hard to be processed to NLP