We identify a binary black hole ( BBH ) merger that appears to be multiply lensed by an intervening galaxy .
The LIGO/Virgo events GW170809 and GW170814 [ 1 ] have indistinguishable waveforms separated by 5 days , and overlap on the sky within the 90 % credible region .
Their strain amplitudes are also similar , implying a modest relative magnification ratio , as expected for a pair of lensed gravitational waves .
The phase of the two events is also consistent with being the same , adding more evidence in support of both events originating from the same BBH merger .
The difference in the published inferred distances of each event can then be interpreted as following from their different magnifications .
The observed chirp masses of both events are also similar , as expected for a pair of lensed events , with a common detected value of 29.1 ^ { +1.3 } _ { -1.0 } M _ { \odot } , lying at the peak of the observed distribution of chirp masses .
We infer this case is a prototypical example of a lensed event that supports our lensing prediction [ 2 ] according to which , cosmologically distant , magnified BBH comprise most of the LIGO/Virgo events with chirp masses enhanced above \simeq 15 M _ { \odot } by the cosmological expansion .
From our predictions we estimate an intrinsic , unlensed , chirp mass of \simeq 10 - 12 M _ { \odot } , with a source redshift in the range 0.9 < z < 2.5 .
We also outline a joint analysis over all baseline permutations that can stringently test our lensing interpretation of these two events .
More generally , lensed events effectively multiply the number of baseline permutations and motivates the use of more interferometers for round the clock coverage of all repeat events of a given source , in order to maximise the orbital details and sky localization of lensed BBH sources .