Its a "southern" way of saying a guy/girl likes you and is flirtatious. When they say or do something to compliment you and it makes you feel good ie: butterflies in your stomach...that's being sweet...
I'd say it is. Repetition is something (an idea, a word, a phrase etc. etc.) which is REPEATED for effect. Here, the word 'sweet' is repeated. It is done for effect. Hank Williams in one of his songs...
That he is sweet on you? You'd have to ask him. Relationships start with honest communication. You'll be able to tell, hopefully, if he's being honest. At the same time, he could be just leading you...