I'm sorry I guess I misunderstood the question.
When you say 'Count of Distinct Records' the formula would be:
Count({Item_No}, {UserID}) - DistinctCount({Item_No}, {UserID})
This does not count Items Nos are appear more than once.
If you want to count all the Iteam Nos in a group (duplicate or otherwise), use this:
Count({Item_No}, {UserID})
If you want to count Disctinct values in the group (meaning a duplicate value would be counted only once), it would be:
DistinctCount({Item_No}, {UserID})
-Abhilash