Counting number of occurences in a time period
Edited based on your sample data.

For an item number to have a total quantity of two or more in a given date range:

 Select MachineRefNo, ItemNumber, Sum(Quantity) NumberOfItems
       From Table
       Where OrderDate Between DateRangeStart And DateRangeEnd        
       Group By MachineRefNo, ItemNumber
       Having Sum(Quantity) &gt; 1


For an item number to have two or more records in a given data range:

Select MachineRefNo, ItemNumber, Count(*) NumberOfOccurances
   From Table
   Where OrderDate Between DateRangeStart And DateRangeEnd        
   Group By MachineRefNo, ItemNumber
   Having Count(*) &gt; 1