Detects duplicate items being added; updated SteamKit2 submodule

This (newly done) pull request adds a check for duplicate item IDs when the user adds an item, which (temporarily) fixes the issue raised in #364 and others until more testing can be done. Should be a stopgap measure and hopefully we can figure out other ways to detect the problem as well.

The SteamKit2 submodule has been updated to point at the latest commit as well.