Goal The same domain should not have multiple CHECK constraints with exactly the same Boolean expression. 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 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 Drop the redundant constraints.
Data Source INFORMATION_SCHEMA only
SQL Query 
WITH checks AS (SELECT dc.domain_schema, dc.domain_name, cc.constraint_name, cc.check_clause
FROM INFORMATION_SCHEMA.domain_constraints AS dc
INNER JOIN INFORMATION_SCHEMA.domains AS d USING (domain_schema, domain_name)
INNER JOIN INFORMATION_SCHEMA.check_constraints AS cc USING (constraint_schema, constraint_name)
WHERE cc.check_clause NOT LIKE '%IS NOT NULL'  AND 
dc.domain_schema NOT IN (SELECT schema_name
FROM INFORMATION_SCHEMA.schemata
WHERE schema_name<>'public' AND
schema_owner='postgres' AND schema_name IS NOT NULL))
SELECT domain_schema, domain_name, check_clause, Count(*) AS number_of_occurrences, string_agg(constraint_name, ';<br>' ORDER BY constraint_name) AS constraints
FROM checks
GROUP BY domain_schema, domain_name, check_clause
HAVING Count(*)>1
ORDER BY Count(*) DESC, domain_schema, domain_name;

SQL statements that help generate fixes for the identified problem.

Fix Action #1
WITH duplicate_checks AS (SELECT dc.domain_schema, dc.domain_name, cc.check_clause,
array_agg(DISTINCT cc.constraint_name) AS constraints_array 
FROM INFORMATION_SCHEMA.domain_constraints AS dc
INNER JOIN INFORMATION_SCHEMA.domains AS d USING (domain_schema, domain_name)
INNER JOIN INFORMATION_SCHEMA.check_constraints AS cc USING (constraint_schema, constraint_name)
WHERE cc.check_clause NOT LIKE '%IS NOT NULL'  AND 
dc.domain_schema NOT IN (SELECT schema_name
FROM INFORMATION_SCHEMA.schemata
WHERE schema_name<>'public' AND
schema_owner='postgres' AND schema_name IS NOT NULL)
GROUP BY dc.domain_schema, dc.domain_name, cc.check_clause
HAVING Count(DISTINCT cc.constraint_name)>1)
SELECT format ('ALTER DOMAIN %1$I.%2$I DROP CONSTRAINT %3$I;', domain_schema, domain_name, unnest(constraints_array)) AS statements
FROM duplicate_checks
ORDER BY domain_schema, domain_name;

Drop the constraint. One of the constraints must stay in place.

SQL Query to Generate FixDescription
WITH duplicate_checks AS (SELECT dc.domain_schema, dc.domain_name, cc.check_clause,
array_agg(DISTINCT cc.constraint_name) AS constraints_array 
FROM INFORMATION_SCHEMA.domain_constraints AS dc
INNER JOIN INFORMATION_SCHEMA.domains AS d USING (domain_schema, domain_name)
INNER JOIN INFORMATION_SCHEMA.check_constraints AS cc USING (constraint_schema, constraint_name)
WHERE cc.check_clause NOT LIKE '%IS NOT NULL'  AND 
dc.domain_schema NOT IN (SELECT schema_name
FROM INFORMATION_SCHEMA.schemata
WHERE schema_name<>'public' AND
schema_owner='postgres' AND schema_name IS NOT NULL)
GROUP BY dc.domain_schema, dc.domain_name, cc.check_clause
HAVING Count(DISTINCT cc.constraint_name)>1)
SELECT format ('ALTER DOMAIN %1$I.%2$I DROP CONSTRAINT %3$I;', domain_schema, domain_name, unnest(constraints_array)) AS statements
FROM duplicate_checks
ORDER BY domain_schema, domain_name;
Drop the constraint. One of the constraints 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:

CHECK constraints

Queries of this category provide information about CHECK constraints.

Domains

Queries of this category provide information about reusable specifications of column properties.

Duplication of implementation elements

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

NameDescription
CHECK constraintsQueries of this category provide information about CHECK constraints.
DomainsQueries of this category provide information about reusable specifications of column properties.
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)