The bottom line is - you should never see this - but organisations need someone who can clean this up.
Usually this happens in companies where everyone is an administrator - which is a great strategy for encouraging creativity, but not so great for building an intuitive workspace. Adobe has many tools for administrators to avoid this situation - but it all comes down to a little governance - usually a naming convention for new metrics, and some care in creating projects for specific roles. When users create and share new projects, the metrics get shared. When you push projects to multiple report suites and save them, or recreate the metrics, then you can end up with more, The best ways to avoid this are: 1) Use the default metrics for what they are intended. In the image above - the first metric, and the third from the bottom are out-of-the-box metrics from Adobe. ALL of the others have been created and shared by someone. If those metrics are not exactly like the OOTB metric, then their name should reflect how they are different. If they are exactly alike, they should be removed. 2) have a "Style Guide" for naming - be clear how AOV might be different in this part of the company and why it needs to be a new metric
3) don't share everything with everyone
4) As administrators - get really good at curating projects - or even better, curate Virtual Report suites so that users don't have to see all these metrics - it's confusing. Curation solves this problem by only showing a group exactly the dimensions, metrics, segments that pertain to them.
5) Use TAGS - all metrics should have tags that explain what group, when to use, what reports.
6) You can search for TAGS when your in a projects and have the list like the one above, by pressing "#" and typing a Tag. For example, perhaps in the list above you are trying to find the "Average Page Views Per Day" for the "Electronics" department. If one of the above metrics were tagged with "Electronics" you could simply type "#Electronics" to display only the metrics for Electronics.
7) As a user when you find the metrics you like, you can flag them as a favorite. (Right Click when you hover over the metrics or dimension components).