"Hoe toevallig" is correct, but barely ever used. If you reader older Dutch texts, you might stumble upon something like that, but in exclamations, the "hoe" is replaced with "wat" nowadays. It's just one of these things that has grown to be widely accepted in everyday language.

"Wat toevallig" is the preferred form (107k google pages), but "hoe toevallig" is a good second (32k pages). It seems to be a more archaic form . My daughter and I often use "hoe" instead of "wat" to indicate sarcasm, but this is just a personal jocular use.