For a surjective function from A--> B, I was just wondering if more than one elements in B can point to the same element in A if the function is surjective.
The object you are describing is not a function. A function from A into B is a set of ordered pairs (a, b) such that a is in A and b is in B and whenever (a, b) and (a, c) are elements of the function, we have b = c. In other words, as a function from R into itself, it does not pass the vertical line test.
It is well known that a vector space always admits an algebraic (Hamel) basis. This is a theorem that follows from Zorn's lemma based on the Axiom of Choice (AC).
Now consider any specific instance of vector space. Since the AC axiom may or may not be included in the underlying set theory, might there be examples of vector spaces in which an Hamel basis actually doesn't exist ?