It is possible to have conflicting results from repeating the same experiment twice. This is the reason that all experiments should be done in triplicates and repeated at least thrice. It is possible that a repeated experiment gives you different/opposite results.
However, please be completely sure that these are genuine results and not due to change of any experimental parameters. Note that you will be contradicting yourself so you need to be careful. Once you are sure of the contradictory results, you should do additional studies to identify why these differences arose and add information to your previous study accordingly. A simple replication of previous results with similar data sets will not qualify for a separate publication.