Goal Find materialized views with exactly the same subquery. There should not be multiple materialized views with the same subquery. Do remember that the same task can be solved in SQL usually in multiple different ways. Thus, the exact copies are not the only possible duplication.
Notes In the returned subquery of materialized view the query replaces each newline character with the line break (br) tag for the better readability in case the query result is displayed in a web browser. In case of the string_agg function, the line break (br) tag is used as a part of the separator for the better readability in case the query result is displayed in a web browser.
Type Problem detection (Each row in the result could represent a flaw in the design)
Reliability High (Few or no false-positive results)
License MIT (opens in new tab)
Fixing Suggestion All but one are redundant. Drop redundant materialized views.
Data Source system catalog only
SQL Query 
SELECT regexp_replace(definition,'[\r\n]','<br>','g') AS definition, mat_views, number_of_materialized_views
FROM (SELECT definition, string_agg(schemaname || '.'|| matviewname,';<br>' ORDER BY schemaname || '.'|| matviewname) AS mat_views, Count(*) AS number_of_materialized_views
FROM pg_catalog.pg_matviews
GROUP BY definition
HAVING Count(*)>1) AS duplicate_mat_views
ORDER BY number_of_materialized_views DESC;

SQL statements that help generate fixes for the identified problem.

Fix Action #1
WITH duplicate_mat_views AS 
(SELECT definition AS mat_view_definition, array_agg(schemaname || '.'|| matviewname) AS mat_views
FROM pg_catalog.pg_matviews
GROUP BY definition
HAVING Count(*)>1) 
SELECT format('DROP MATERIALIZED VIEW %1$s;', unnest(mat_views)) AS statements
FROM duplicate_mat_views
ORDER BY mat_view_definition;

Drop the materialized view. One of the materialized views must stay in place.

SQL Query to Generate FixDescription
WITH duplicate_mat_views AS 
(SELECT definition AS mat_view_definition, array_agg(schemaname || '.'|| matviewname) AS mat_views
FROM pg_catalog.pg_matviews
GROUP BY definition
HAVING Count(*)>1) 
SELECT format('DROP MATERIALIZED VIEW %1$s;', unnest(mat_views)) AS statements
FROM duplicate_mat_views
ORDER BY mat_view_definition;
Drop the materialized view. One of the materialized views must stay in place.

Collections

This query belongs to the following collections:

Find problems automatically

Queries, that results point to problems in the database. Each query in the collection produces an initial assessment. However, a human reviewer has the final say as to whether there is a problem or not .

NameDescription
Find problems automaticallyQueries, that results point to problems in the database. Each query in the collection produces an initial assessment. However, a human reviewer has the final say as to whether there is a problem or not .

Categories

This query is classified under the following categories:

Comfortability of data management

Queries of this category provide information about the means that have been used to make the use or management of database more comfortable and thus, more efficient.

Data redundancy

Queries of this category provide information about possible data redunancy.

Derived tables

Queries of this category provide information about the derived tables (views, materialized views), which are used to implement virtual data layer.

Duplication of implementation elements

Queries of this catergory provide information about the duplication of the database objects.

NameDescription
Comfortability of data managementQueries of this category provide information about the means that have been used to make the use or management of database more comfortable and thus, more efficient.
Data redundancyQueries of this category provide information about possible data redunancy.
Derived tablesQueries of this category provide information about the derived tables (views, materialized views), which are used to implement virtual data layer.
Duplication of implementation elementsQueries of this catergory provide information about the duplication of the database objects.

Further reading and related materials:

https://refactoring.guru/smells/alternative-classes-with-different-interfaces
https://refactoring.guru/smells/duplicate-code
The corresponding code smell in case of cleaning code is "G5: Duplication". (Robert C. Martin, Clean Code)
Reference
https://refactoring.guru/smells/alternative-classes-with-different-interfaces
https://refactoring.guru/smells/duplicate-code
The corresponding code smell in case of cleaning code is "G5: Duplication". (Robert C. Martin, Clean Code)