If you are trying to compare the current value of HH_Number to one that was previously entered that won't work because only case is loaded at a time.
If HH_Number is an id-item then you could use the keylist() function to get the list of previous cases, extract the HH_Number from each key and compare it to the HH_Number just entered.
If HH_Number is not an id-item then you would need to write the HH_Number to an external data file each time it is entered using writecase and then use loadcase to check to see if each new one is a duplicate.
You could also consider setting HH_Number to be auto-increment in which case it will automatically be increased by one for each household. This way it will be unique.
Finally if HH_Number is your ONLY id variable then you don't need to anything. CSPro will give an error if it is not unique.