When I import opportunities into my Zoho CRM account, I map the "Opportunities Id" field to a column in an Excel file that lists my organization's own "Project ID" for each of our projects. 

However, Zoho CRM doesn't recognize this Project ID and instead assigns its own unique ID in the "Opportunities Id" column to each record (e.g. zcrm_818539000000112561). In other words, when I export, none of my organization's unique IDs appear, and each record has it's own Zoho-generated unique ID. Thus, each time I import an updated database of project progress, it duplicates every record, even though multiple opportunities have the same name in the "Opportunities Name" column.

In essence, how do I get Zoho CRM to import records and overwrite them based on my organization's unique record IDs (and prevent Zoho CRM from duplicating these records)?