Provision Unique site Column

Make sure to include the highlighted properties. 

<Field ID="{E9ADC1A3-C044-4C99-AB48-AFAD5F657648}"
  Group ="Custom"
  DisplayName="Id Number"
  Name="CCWIdNumber"
  Type="Text"
  EnforceUniqueValues="TRUE"
  Indexed="TRUE"
  Required="TRUE"
  ShowInDisplayForm="TRUE"
  ShowInNewForm="TRUE"
  ShowInEditForm="TRUE" />

One thing to note is that you can't share this field in a document set
if you've found a way please let me know.