In a very real way it was. There were two completely geographically separate instruments which recorded the same signal with a delay that is consistent with the light travel time between them.

In the future this will get better when VIRGO in Italy and KAGRA [2] in Japan come online. Then we will have 4 independent detectors which will be able to verify that same signal is observed at the same time.

Obviously of course given the transient nature of what is being observed once the merger has occurred it will very rapidly stop producing gravitational waves so we will not be able to measure the same event again.

[1]: https://en.wikipedia.org/wiki/Virgo_interferometer

[2]: https://en.wikipedia.org/wiki/KAGRA