Multiple instances of installed applications and updates displayed in updates history
If you get it working just by redownloading, it's nothing to worry about. 

As both a user and developer I can tell that these types of errors aren't uncommon (Often times caused by an unstable internet connection or if the Mac has been on standby for a longer period whereafter the download is continued). But it's decided by multiple factors including your router, signal strength, geographical location and so on.

The problem is that it can be hard to perfectly download every little piece of a large file. Sometimes it's read wrong or written wrong to the disk and this causes corruption and possibly confuses the part of App Store that displays progress and download items.

To Apple's credit, though, they seem to be able to "chunk" download effectively most of the time, where other apps (like Chrome for instance) just kills the download and makes you manually start over. 
Personally, I rarely have these types of errors anymore (they were much more common in the "early days" of the App Store).