Membership export includes multiple copies of the same membership

This is currently easy to reproduce on Demo but may relate to config so may disappear on refresh (I tried to copy what customer had without having figured out the problem source)

1) Do an advanced search - choose 'Adams' as the group & show as membership = 6 results - there are 10 members in the group

2) export members using saved field mapping 'membership'

3) 9 rows rather than 6 are exported

Note that we normally do this from the smart group - ie. manage groups, go to group, edit smart group criteria, view as members, export from there - same result

The export mapping seems to matter & I have home phone & mobile in there - pic attached

UPDATE removing one of the phones on customer site seemed to cause export to export correct number of rows