Detects duplicate items being added; also fixes nuget error after cloning

This 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.

Also fixes a nuget error relating to SteamKit which I got upon opening the solution.