Two black holes in a collection of stars known as a globular cluster are not unique - another black hole candidate has been found in globular cluster M62.
The globular cluster M62 is located in the constellation Ophiuchus, about 22,000 light years from Earth. Black holes are stars that have died, collapsed into themselves and now have such a strong gravitational field that not even light can escape from them.
Until recently, astronomers had assumed that the black holes did not occur in globular clusters, which are some of the oldest and densest collections of stars in the universe. Stars are packed together a million times more closely than in the neighborhood of our sun.