Original ticket http://projects.scipy.org/numpy/ticket/236 on 2006-08-07 by trac user martin_wiechert, assigned to unknown.

.reduceat does not handle repeated indices correctly. When an index is repeated the neutral element of the operation should be returned. In the example below [0, 10], not [1, 10], is expected.

In [1]:import numpy

In [2]:numpy.version.version
Out[2]:'1.0b1'

In [3]:a = numpy.arange (5)

In [4]:numpy.add.reduceat (a, (1,1))
Out[4]:array([ 1, 10])