Use verification rule to stop input of duplicate values in a field of another form?

Karl_Donaubauer 

Karl,

Thank you very much for your input. I tried what you suggested. However, I keep getting an error where Access is telling me it cannot find the respective field "JOB". As the actual field name in the table is "JOB#", I am wondering if it does not recognize the "#" character. I could try changing the field name in the Sharepoint table to what you have in your code as "JobNo". However, I fear this may break other relationships within the database after update. Here is the code I entered in the BeforeUpdate area of the textbox control.

If Not IsNull(Dlookup("JOB#", "JOB_TABLE", _
    "JOB# = " & Me!JOB#)) _
    And Me!JOB# <> nz(Me!JOB#.OldValue) Then

    MsgBox Me!JOB# & " already exists.", vbOKOnly, "Duplicate!"
    Cancel = True

End If

Thoughts?