Prevent creation of duplicate not-null constraints for domains

This was previously harmless, but now that we create pg_constraint rows
for those, duplicates are not welcome anymore.

Backpatch to 18.

Co-authored-by: jian he <jian.universality@gmail.com>
Co-authored-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/CACJufxFSC0mcQ82bSk58sO-WJY4P-o4N6RD2M0D=DD_u_6EzdQ@mail.gmail.com

src/backend/commands/typecmds.c

@@ -939,11 +939,19 @@ DefineDomain(ParseState *pstate, CreateDomainStmt *stmt)
 				break;
 
 			case CONSTR_NOTNULL:
-				if (nullDefined && !typNotNull)
+				if (nullDefined)
+				{
+					if (!typNotNull)
+						ereport(ERROR,
+								errcode(ERRCODE_SYNTAX_ERROR),
+								errmsg("conflicting NULL/NOT NULL constraints"),
+								parser_errposition(pstate, constr->location));
+
 					ereport(ERROR,
-							errcode(ERRCODE_SYNTAX_ERROR),
-							errmsg("conflicting NULL/NOT NULL constraints"),
+							errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+							errmsg("redundant NOT NULL constraint definition"),
 							parser_errposition(pstate, constr->location));
+				}
 				if (constr->is_no_inherit)
 					ereport(ERROR,
 							errcode(ERRCODE_INVALID_OBJECT_DEFINITION),

src/test/regress/expected/domain.out

@@ -1019,6 +1019,11 @@ insert into domain_test values (1, 2);
 -- should fail
 alter table domain_test add column c str_domain;
 ERROR:  domain str_domain does not allow null values
+-- disallow duplicated not-null constraints
+create domain int_domain1 as int constraint nn1 not null constraint nn2 not null;
+ERROR:  redundant NOT NULL constraint definition
+LINE 1: ...domain int_domain1 as int constraint nn1 not null constraint...
+                                                             ^
 create domain str_domain2 as text check (value <> 'foo') default 'foo';
 -- should fail
 alter table domain_test add column d str_domain2;

src/test/regress/sql/domain.sql

@@ -602,6 +602,9 @@ insert into domain_test values (1, 2);
 -- should fail
 alter table domain_test add column c str_domain;
 
+-- disallow duplicated not-null constraints
+create domain int_domain1 as int constraint nn1 not null constraint nn2 not null;
+
 create domain str_domain2 as text check (value <> 'foo') default 'foo';
 
 -- should fail