fix cols and distinct conflicts (go-xorm#927)

session_cols.go

@@ -47,6 +47,14 @@ func (m columnMap) contain(colName string) bool {
 	return false
 }
 
+func (m *columnMap) add(colName string) bool {
+	if m.contain(colName) {
+		return false
+	}
+	*m = append(*m, colName)
+	return true
+}
+
 func setColumnInt(bean interface{}, col *core.Column, t int64) {
 	v, err := col.ValueOf(bean)
 	if err != nil {

statement.go

@@ -624,7 +624,7 @@ func (statement *Statement) Select(str string) *Statement {
 func (statement *Statement) Cols(columns ...string) *Statement {
 	cols := col2NewCols(columns...)
 	for _, nc := range cols {
-		statement.columnMap = append(statement.columnMap, nc)
+		statement.columnMap.add(nc)
 	}
 
 	newColumns := statement.colmap2NewColsWithQuote()

statement_test.go

@@ -10,6 +10,7 @@ import (
 	"testing"
 
 	"github.com/go-xorm/core"
+	"github.com/stretchr/testify/assert"
 )
 
 var colStrTests = []struct {
@@ -180,3 +181,25 @@ func createTestStatement() *Statement {
 	}
 	return nil
 }
+
+func TestDistinctAndCols(t *testing.T) {
+	type DistinctAndCols struct {
+		Id   int64
+		Name string
+	}
+
+	assert.NoError(t, prepareEngine())
+	assertSync(t, new(DistinctAndCols))
+
+	cnt, err := testEngine.Insert(&DistinctAndCols{
+		Name: "test",
+	})
+	assert.NoError(t, err)
+	assert.EqualValues(t, 1, cnt)
+
+	var names []string
+	err = testEngine.Table("distinct_and_cols").Cols("name").Distinct("name").Find(&names)
+	assert.NoError(t, err)
+	assert.EqualValues(t, 1, len(names))
+	assert.EqualValues(t, "test", names[0])
+}