When HA_DUPLICATE_POS is not supported, the row to replace was navigated by
ha_index_read_idx_map, which uses only hash to navigate.

Suchwise, given a hash collision it may choose an incorrect row.

handler::position would be correct and very convenient to use here.

dup_ref is already set by handler independently of the engine
capabilities, when an extra lookup is made (for long unique or something else,
for example WITHOUT OVERLAPS) such error will be indicated by
file->lookup_errkey != -1.