fix regex and add test

arduino/builder/sketch.go

@@ -28,7 +28,7 @@ import (
 	"github.com/pkg/errors"
 )
 
-var includesArduinoH = regexp.MustCompile(`^\s*#\s*include\s*[<\"]Arduino\.h[>\"]`)
+var includesArduinoH = regexp.MustCompile(`(?m)^\s*#\s*include\s*[<\"]Arduino\.h[>\"]`)
 
 // QuoteCppString returns the given string as a quoted string for use with the C
 // preprocessor. This adds double quotes around it and escapes any

arduino/builder/sketch_test.go

@@ -19,6 +19,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/arduino/arduino-cli/arduino/builder"
@@ -107,3 +108,13 @@ func TestMergeSketchSources(t *testing.T) {
 	assert.Equal(t, 2, offset)
 	assert.Equal(t, string(mergedBytes), source)
 }
+
+func TestMergeSketchSourcesArduinoIncluded(t *testing.T) {
+	s, err := builder.LoadSketch(filepath.Join("testdata", t.Name()), "")
+	assert.Nil(t, err)
+	assert.NotNil(t, s)
+
+	// ensure not to include Arduino.h when it's already there
+	_, source := builder.MergeSketchSources(s)
+	assert.Equal(t, 1, strings.Count(source, "<Arduino.h>"))
+}

arduino/builder/testdata/TestMergeSketchSourcesArduinoIncluded/TestMergeSketchSourcesArduinoIncluded.ino

+// a comment
+
+   #    include		<Arduino.h>
+
+void setup() {
+}
+
+void loop() {
+}