The error description says it found more than one record identical to the one you are trying to update, and because you want to update only one record to the recordset at a time (non-bulk update) it prevents a mistake from happening.
When a record is to be updated, ADO needs to know where it is. It does this by using the assigned primary or unique key and then does a search in the table for it. If there isn't a unique key, then it tries to identify the record by finding matching uniqness across all fields. If it still cannot find a unique record, you will get this error.
Be sure to add and include a primary or unique field, or combination of fields (Main object ID and secondary Object ID together to provide a unique key where no other record has this combination) to your table and include it in the recordset - even if it is just an Auto Number field not being used for anything else.