If you compliment a girl and she returns one, is that a legitimate ioi? A lot of the time when I give a girl a compliment they will either say thankyou or give that false modesty bullshit. There doesnt seem to be any social rule or etiquette which says a compliment must be returned. So when a girl does return the compliment is that an indicator of interest or politeness? How would you react if a hot girl complimented you verses an ugly girl?

Also does it matter if its the same compliment that is returned, for example:

Guy: You smell nice
Girl: thanks, you smell nice too

or

Guy: I like your hair
Girl: thanks, you have nice eyes

Is the second situation more genuine?

Thoughts, ideas, experiences?

A returned compliment can be an IOI, but it can also be nothing more than politeness. At least it means that the girl find you polite/nice. It doesn't have to mean anything more. In you first example where the girl returns the same compliment, it is most likely so because it's the easiest thing to do, almost like a reflex. If she gives you a different compliment in return, it is almost certainly genuine since it takes more effort to come up with it - e.g. she has been thinking about your eyes.