Option to tell customer if they have already borrowed a title
An option added in the patron registration that turns on a "loan tracking" feature. When checking out books, if this feature is turned on, then when a barcode is scanned that the patron has already borrowed a message comes up to let the patron know they have already read this title.
Would check at a bib level (so not just the actual item that is scanned, but any bib that item is associated with). Should be based on what is in the reading history of the account. Would need enhancement for serial issues, as the reading history currently only advises the serial title previously borrowed and not the issue.
We had this in our last LMS and it was a huge value add to our customers.

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Lynn Reynish commented
We've developed a similar feature for use in the customer's My Account in our custom OPAC (it highlights bibliographic records that customers have already checked out). We *really* notice the shortcomings of the reading history feature here even at the bib level.
What we would like is for the reading history to change what is stored and how. On a previous ILS (Dynix Horizon) the reading history didn't store the bibliographic record number or the item record number or the barcode. It stored a set number of characters from the MARC 245 (enough to capture the title and the author from most works). This allows cross material type matches (because if you've read the title, you probably don't want to listen to the audiobook as its the same content). Further, this table isn't touched when bibliographic records are deleted - like Polaris' reading history is.
I think the requested enhancement would be nice for customers that don't have developers or access to their own custom OPAC but for it to work best the way the reading history works needs to be adjusted first.