Holdings - Two B
What is this?
Call numbers in holdings records are stored in 852 $h and $i. The sublocation/collection is stored in $b.
When is this trouble?
If your library does typically not use repeated 852 subfield $b to indicate sublocations or collections, having an 852 with two $b might be a sign that a call number has been copied in from a bibliographic record without updating the code (in bibliographic records, schemes like Library of Congress and National Library of Medicine call numbers are stored in $a and $b of their bibliographic fields)
If part of the call number is stored in a second $b instead of a $i, that part of the call number may not show up in online catalog display, and may not have even made it to the spine label of the book.
How to Find
If you are in Alma, there is an analysis to search for this issue in the "Looking for Trouble" folder. Go to the "Holdings" folder and look for the "2 Subfield B" analysis.