"Duplicate entry" SQL warning with Gmap Taxonomy Markers

I receive the following warning when changing/setting a marker symbol on the Taxonomy Term edit page (Gmap 5.x-1.0-rc2 and Location 5.x-3.0-rc2):

user warning: Duplicate entry '44' for key 1 query: INSERT INTO gmap_taxonomy_node (nid, tid, marker) (SELECT t.nid, t.tid, g.marker FROM term_node t INNER JOIN gmap_taxonomy_term g ON t.tid = g.tid WHERE t.tid = 255) in C:\www\includes\database.mysql.inc on line 174.

The "term_data_tid" value in the "sequences" table matches the larges TID value in the "term_data" table.
The same is true for the NID value in "node."

I don't know what caused the problem and how to find/fix the duplicate entry. Despite the warning, everything seems to be working fine.

Any help would be very welcome.