If you select the Catalog module in step 1 of a guided report you will see 'Koha itemtype' appear twice in step 4. 

To recreate:
-Build a guided report
-In step 1 select 'Catalog' as the module
-In step 3 add at least one column
-In step 4 notice that 'Koha itemtype' displays twice no matter what you've selected in step 3
-Check both the 'Koha itemtype' and continue to build your report.
-On the confirm your report page notice the SQL looks like this:

 SELECT items.itemnumber FROM items LEFT JOIN biblioitems on (items.biblioitemnumber=biblioitems.biblioitemnumber) LEFT JOIN biblio on (biblioitems.biblionumber=biblio.biblionumber) WHERE items.itype='BK' AND items.itype='BK' 


No matter what you select as the 2nd itype you will get a duplicate of what you selected in the first:

WHERE items.itype='BK' AND items.itype='BK'