By the same token, there is a type of duplicate index that can provide huge performance increases.  They usually (but not always) are considered to be
"Covering Indexes".  You can have the "perfect" Clustered Index for your query but, unless you're using a huge number of columns from a very wide table in your query, a much narrower Non-Clustered Index with the exact same key as the Clustered Index and the correct INCLUDES (think of it as being very similar to a much smaller Clustered Index) can make the queries that use the Non-Clustered Index absolutely fly compared to when the query uses the Clustered Index simply because of the greatly reduced page count thanks to more rows per page in the Non_Clustered Index.