an error message that I don't understand

I get this error message:

table tid from new index tuple (32586,21) overlaps with invalid duplicate tuple at offset 120 of block 4398 in index "aacorrections_tcpreading_idx"

from an update query of the
update table 1 set columnx = ‘yes’
where table1.xmlid in (select xmlid from table2)

I have no idea what is going on or how to deal with it and will be grateful for advice