It seems obvious to me that a small string sequence could be identical from two differents long original texts. Even it happend, the hash function is NOT the original message, and a collision could happen. It does'nt mean that the two original texts are the same.
Am i right ?
Yes. A hash is nothing more than a function mapping data of arbitrary size to an output of fixed, smaller size so by definition you can always construct two inputs which yield the same hash. What makes crypto hashes secure is that this is normally very, very hard to do - that is, given a hash generate an input from it.