Item Product Rule not Working if more than 1 Item is created

Summary: We have a product rule which populates EFF value with Item Number generated if blank as below:-

Primary If Expression = isNull([Item].[CRC/UPC DATA].[CRC]) and ([Item].[Item Basic].[Item] != "XXXXXX")

Then Expression= True

Return Value = [Item].[Item Basic].[Item]

This rules works without issue if we are creating one single Item instance but if we create more than one Item the value of attribute is getting populated as 'Sequence generated 1', 'Sequence generated 2' as when creating multiple Items default value in Item field is 'Sequence generated ...' . How can I modify the above rule so it takes in consideration of multiple Item creation