Rule Definition
It is useless to have several indexes on the same column(s) or to have an index on columns already covered by a composite index. This will not improve read performance and will decrease update/insert/delete performance because of index maintenance.

Remediation
Drop the indexes that are not necessary.

Violation Code Sample

-- SQL Sample
create table MyTable (C1 int identity, C2 int)

create index MyTable_I1 on MyTable(C1)
create index MyTable_I2 on MyTable(C1)
create index MyTable_I3 on MyTable(C1, C2)

Fixed Code Sample

-- SQL Sample

create table MyTable (C1 int identity, C2 int)

-- create index MyTable_I1 on MyTable(C1)
-- This index is useless as it is the same as MyTable_I2.

-- create index MyTable_I2 on MyTable(C1)
-- This index is nearly useless, as queries using C1 will be covered by the index MyTable_I3. It can then be dropped.

create index MyTable_I3 on MyTable(C1, C2)

https://www.mssqltips.com/sqlservertip/1449/avoid-index-redundancy-in-sql-server-tables/