Make sure we don't overwrite existing ENV vars.

godotenv.go

@@ -34,7 +34,7 @@ func loadFile(filename string) (err error) {
 		if !isIgnoredLine(fullLine) {
 			key, value, err := parseLine(fullLine)
 
-			if err == nil {
+			if err == nil && os.Getenv(key) == "" {
 				os.Setenv(key, value)
 			}
 		}

godotenv_test.go

@@ -83,6 +83,16 @@ func TestLoadQuotedEnv(t *testing.T) {
 	loadEnvAndCompareValues(t, envFileName, expectedValues)
 }
 
+func TestActualEnvVarsAreLeftAlone(t *testing.T) {
+	os.Clearenv()
+	os.Setenv("OPTION_A", "actualenv")
+	_ = Load("fixtures/plain.env")
+
+	if os.Getenv("OPTION_A") != "actualenv" {
+		t.Error("An ENV var set earlier was overwritten")
+	}
+}
+
 func TestParsing(t *testing.T) {
 	// unquoted values
 	parseAndCompare(t, "FOO=bar", "FOO", "bar")