When we update an issue's custom field we are getting error

[shanthini.g]

When we update an issue's custom field we are getting error like below
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Database query failed. Error received from database was #1062: Duplicate entry '51-154543' for key 'PRIMARY' for the query: UPDATE mantis_custom_field_string_table
SET value=?
WHERE field_id=? AND
bug_id=?.

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But we do not have duplicate values. So not sure what was the issue. Please help.

Thanks

[cas]

did you check the database itself?