You might conclude that running an experiment for the second or third time is boring and produces no new information. That's not so. The only way to figure out whether a particular result really is scientific truth is by repeating experiments, in different places and times, with different equipment and measurement devices, under the guidance of different scientists. But beyond the effort of obtaining scientific "certainty" one may also find out new things. It is always possible that the original experiment was influenced by something that the experimentalist was not aware of, so repeating the experiment may reveal the need for more a more precise statement of the experimental set up. (For instance, minute impurities in water can cause large changes in its resistance to electricity. Pure water is not a good conductor. If two measurements of water's conductivity give different values, this may reflect need for more control over the water's impurities rather than a fundamental scientific inacuracy.) Repeating an experiment may bring these set up issues to light, as one goes through a trial and error struggle to reproduce the original result.
In addition, even if the first experiment's outcome was observed correctly, the mechanisms which produced that outcome may have been misunderstood. Repetition with differently constructed experimental setups, or with a variety of measuring methods, gives evidence for a robust understanding of how the experiment works. Conversely, a different result obtained using slightly different equipment may be the first step toward a deeper understanding of what's going on.