The following compiles without issues:

contract C {
  function f() {
    int a = a;
  }
}

Since variables are pre initialised to 0, the above should be effectively a no-op. I do think however it can be confusing. I would suggest throwing an error in this case or at a bare minimum a warning.

Think about:

  int aa = 5;
  int a = 4 * a; // user intended to use aa

The same applies to var, though that is already rejected (for a different reason):

2988.sol:3:13: Error: Declaration referenced before type could be determined.
    var a = a;
            ^