I'll give a shot at why this implies that H is not a secure hash function, though I could be wrong. The outputs of a secure hash function should be randomly distributed across the set of all possible outputs. If H is a secure hash function that outputs a value in the set {0,1}^128 (a 128 bit output), then H(a) and H(a+a) should both be 128 bit outputs whose mapping are indistinguishable from values chosen truly at random from this set. Although there is a discernable pattern in the two inputs to H (the concatenation), there should be no discernable pattern in the two outputs of H that you could use to reliably map H(a) to H(a+a) for any given a.