In the After Update event of your field put code like this:
Me![FieldName].Tag = Me![FieldName].Value
In the Double Click Event of the same field put code like this:
If Not (IsNull(Me![FieldName].Tag) Or Me![FieldName].Tag = "") Then
Me![FieldName].Value = Me![FieldName].Tag
End If
Just change FieldName to the name of your field.
HTH
Dave