Like other mainstream commercial database systems, Microsoft SQL Server allows you to create multiple indexes on the same column of a table. This increases the likelihood of having duplicate indexes in the database because SQL Server does not prevent you from creating duplicate indexes, infect we can create up to 999 duplicate indexes on each table inside database. Having duplicate indexes on tables columns can significantly hurt the performance of your database because SQL Server has to maintain each duplicate index separately (such as updating these duplicate indexes during DML operations and calculating and updating statistics for these duplicate indexes). Moreover, SQL Server query optimizer will consider each of them when it optimizes queries, which can cause serious performance impact.
Check out my article here in which you will learn about what duplicate index is, how they are created, and what system catalogs we can use to find and remove these duplicate indexes from SQL Server databases.
This article is published on SQL-SERVER-PERFORMANCE.COM.