Try this:
1)Select A1:A10 (with A1 as the active cell)
2)DataValidation
Allow: Custom
Formula: =COUNTIF(A$1:A1,A1)=1

Then, with A15:A20 (A15 active)
DataValidation
Allow: Custom
Formula: =COUNTIF(A$15:A20,A15)=1

Those will allow any entry in the lists, but no duplicates.

Does that help?

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Regards,
Ron