Buddycards 2.5.2 has a dupe glitch that is fixed by updating to the latest version. More information can be found at https://www.curseforge.com/minecraft/mc-mods/buddycards/files/3576087

A stopgap measure for server admins who run into this issue in the meantime is to replace the /$packagepath/container/BinderContainer.class files with the ones from 2.5.3. It's incredibly janky but it seems to be working fine on my A&B server, and still allowing 2.5.2 clients to connect.