Duplicate Display Value Errors

When I try import a form into FormsCentral I get an error saying "cannot import - Duplicate Display Values found" and it gives the field name where the error is.

The field is a dropdown list consisting of a blank space, 8, 9, 10, 11. 

I have the same field on the three pages of the document.

I have tried "commit values immediately" and deleting the space but still ge the same error.

Pelase help.

Thanks

Harry