Improve wording of error message added in commit 714805010.

Per suggestions from Peter Eisentraut and David Johnston.
Back-patch, like the previous commit.

Discussion: https://postgr.es/m/E1dv9jI-0006oT-Fn@gemulon.postgresql.org

src/backend/commands/analyze.c

@@ -396,7 +396,7 @@ do_analyze_rel(Relation onerel, int options, VacuumParams *params,
 			if (bms_is_member(i, unique_cols))
 				ereport(ERROR,
 						(errcode(ERRCODE_DUPLICATE_COLUMN),
-						 errmsg("column \"%s\" of relation \"%s\" is specified twice",
+						 errmsg("column \"%s\" of relation \"%s\" appears more than once",
 								col, RelationGetRelationName(onerel))));
 			unique_cols = bms_add_member(unique_cols, i);
 

src/test/regress/expected/vacuum.out

@@ -92,7 +92,7 @@ VACUUM (FULL) vacparted;
 VACUUM (FREEZE) vacparted;
 -- check behavior with duplicate column mentions
 VACUUM ANALYZE vacparted(a,b,a);
-ERROR:  column "a" of relation "vacparted" is specified twice
+ERROR:  column "a" of relation "vacparted" appears more than once
 ANALYZE vacparted(a,b,b);
-ERROR:  column "b" of relation "vacparted" is specified twice
+ERROR:  column "b" of relation "vacparted" appears more than once
 DROP TABLE vacparted;