It happened to me a number of times that by mistake I bought records that I already had. To avoid this I made a list of all my records, however, it has not been updated during the last few years, so now I am taking risks again.
With really expensive items, I double check. When finances are low I´m extra careful.
When I buy over the Internet I take time to check. When at a record fair or in a shop, you sometimes rush into things.
Find it hard to memorize the titles of records. But for LPs, I find it quite easy to remember (visualize) what the LP looks like, the cover art (only problem when there are different versions/issues of same album with alternative covers).
/Melismo