scbuild: error on duplicate columns in ALTER PRIMARY KEY

This check has been missing from the declarative schema changer builder.
This would result in execution errors in later stages when the new index
makes it into the table descriptor. Since these are internal errors, the
user experience is correspondingly poor. This patch fixes this.

Fixes #91301

Release note (bug fix): fixed a bug present only in earlier 22.2 release
candidates, in which an ALTER PRIMARY KEY USING COLUMNS (x, x) statement
would result in an internal error instead of the expected user-facing
error with a pg-code.

pkg/sql/logictest/testdata/logic_test/alter_primary_key

@@ -4,6 +4,9 @@ CREATE TABLE t (x INT PRIMARY KEY, y INT NOT NULL, z INT NOT NULL, w INT, INDEX
 statement ok
 INSERT INTO t VALUES (1, 2, 3, 4), (5, 6, 7, 8)
 
+statement error pgcode 0A000 .* contains duplicate column \"y\"
+ALTER TABLE t ALTER PRIMARY KEY USING COLUMNS (y, y)
+
 statement ok
 ALTER TABLE t ALTER PRIMARY KEY USING COLUMNS (y, z)
 

pkg/sql/schemachanger/scbuild/internal/scbuildstmt/alter_table_alter_primary_key.go

@@ -206,6 +206,7 @@ func checkForEarlyExit(b BuildCtx, tbl *scpb.Table, t alterPrimaryKeySpec) {
 		panic(err)
 	}
 
+	usedColumns := make(map[tree.Name]bool, len(t.Columns))
 	for _, col := range t.Columns {
 		if col.Column == "" && col.Expr != nil {
 			panic(errors.WithHint(
@@ -217,6 +218,11 @@ func checkForEarlyExit(b BuildCtx, tbl *scpb.Table, t alterPrimaryKeySpec) {
 				"use columns instead",
 			))
 		}
+		if usedColumns[col.Column] {
+			panic(pgerror.Newf(pgcode.FeatureNotSupported,
+				"new primary key contains duplicate column %q", col.Column))
+		}
+		usedColumns[col.Column] = true
 
 		colElems := b.ResolveColumn(tbl.TableID, col.Column, ResolveParams{
 			IsExistenceOptional: false,