Hi Bruce,
There's a simple work around for this.
If there are duplicate document numbers in the file, and you select any one of them, then the Finder on document number will position you to the first customer with that number, regardless of which one you selected.
If, after selecting the client you want, you clear the document number by pressing "Delete" before pressing the "Enter" key (or clicking the OK button), then you will be positioned to the customer you have selected off the Finder.
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Andrew Bates