[App_Table_Configuration] Duplicate Entry "xxxx" is not allowed as duplicate . . .

[App_Table_Configuration] Duplicate Entry "xxxx" is not allowed as duplicate . . .

I went to switch a "Single Line" Text entry field to "Import Data" and configured all of the parameters including '"Show Link" and "Enable bidirectional relationship" and so forth. But now I'm totally hung up with an error being generated that says:

[App_Table_Configuration] Duplicate entry '1262029000000082829' is not allowed as duplicate entries already exist.

I cannot find any duplicate entries in any of my forms or in the Imported Data. Thanks to this interruption, I've lost my place in my scripting tasks.

What's up??