Go to Record/Request/Page [First]
Set Variable [$$ID; Value:"Table Name"::(field)]
Go to Record/Request/Page [Next]
Loop
If ["Table Name"::(field) = $$ID]
Delete Record/Request [No dialog]
Else
Set Variable [$$ID; Value:"Table Name"::(field)]
Go to Record/Request/Page [Next; Exit after last]
End If
End Loop

via Removing duplicate records from FileMaker « Macsolvers Blog.