You won't hear this usage often. It is idiomatic and confusing. It is said again for a greater effect. A better way of saying it would be: I miss you; oh, but I miss you. Even better: I miss you - oh, how I miss you! or I miss you - I mean, I really miss you!

I dont this really makes much sense in english... where did you hear it??

the second one has idiomatic meaning, Like this: I miss you when you go to Hong Kong. Can I go with you? ( feel lonely because you are not here ) but not surely, the first one would be its real meaning (absence of sth/sb!) --------------------------------------------------------------------------------------------------------------------------------------------------