diff --git a/fftest/test_case.go b/fftest/test_case.go
index 6a7bde9..0760579 100644
--- a/fftest/test_case.go
+++ b/fftest/test_case.go
@@ -59,6 +59,7 @@ func (tc *ParseTest) Run(t *testing.T) {
 			parseFunc = fftoml.Parse
 		case ".env":
 			parseFunc = ffenv.Parse
+			opts = append(opts, ff.WithEnvVars())
 		default:
 			parseFunc = ff.PlainParser
 		}
diff --git a/flag_set_test.go b/flag_set_test.go
index 928c6d7..e96c5f3 100644
--- a/flag_set_test.go
+++ b/flag_set_test.go
@@ -295,6 +295,19 @@ func TestFlagSet_invalid(t *testing.T) {
 		_ = fs.Bool('a', "apple", "this should panic")
 	})
 
+	t.Run("same fold short name", func(t *testing.T) {
+		defer func() {
+			if x := recover(); x == nil {
+				t.Logf("have not expected panic (%v)", x)
+			} else {
+				t.Errorf("want peace, have panic")
+			}
+		}()
+		fs := ff.NewFlagSet(t.Name())
+		_ = fs.Bool('a', "alpha", "this should be OK")
+		_ = fs.Bool('A', "apple", "this should be OK")
+	})
+
 	t.Run("duplicate long name", func(t *testing.T) {
 		defer func() {
 			if x := recover(); x == nil {
diff --git a/parse.go b/parse.go
index bdfb2c7..0b8a03c 100644
--- a/parse.go
+++ b/parse.go
@@ -38,20 +38,23 @@ func parse(fs Flags, args []string, options ...Option) error {
 		option(&pc)
 	}
 
-	// Index valid flags by env var key, to support .env config files (below).
-	env2flag := map[string]Flag{}
-	{
-		if err := fs.WalkFlags(func(f Flag) error {
-			for _, name := range getNameStrings(f) {
-				key := getEnvVarKey(name, pc.envVarPrefix)
-				if existing, ok := env2flag[key]; ok {
-					return fmt.Errorf("%s: %w (%s)", getNameString(f), ErrDuplicateFlag, getNameString(existing))
+	var env2flag map[string]Flag
+	if pc.envVarEnabled {
+		// Index valid flags by env var key, to support .env config files (below).
+		env2flag = map[string]Flag{}
+		{
+			if err := fs.WalkFlags(func(f Flag) error {
+				for _, name := range getNameStrings(f) {
+					key := getEnvVarKey(name, pc.envVarPrefix)
+					if existing, ok := env2flag[key]; ok {
+						return fmt.Errorf("%s: %w (%s)", getNameString(f), ErrDuplicateFlag, getNameString(existing))
+					}
+					env2flag[key] = f
 				}
-				env2flag[key] = f
+				return nil
+			}); err != nil {
+				return err
 			}
-			return nil
-		}); err != nil {
-			return err
 		}
 	}
 
diff --git a/parse_test.go b/parse_test.go
index 62f9b3e..70750ba 100644
--- a/parse_test.go
+++ b/parse_test.go
@@ -2,6 +2,7 @@ package ff_test
 
 import (
 	"embed"
+	"errors"
 	"flag"
 	"os"
 	"path/filepath"
@@ -428,3 +429,30 @@ func TestParse_stdfs(t *testing.T) {
 		t.Errorf("foo: want %q, have %q", want, have)
 	}
 }
+
+func TestParse_shortSameCase(t *testing.T) {
+	t.Parallel()
+
+	newFS := func() *ff.FlagSet {
+		fs := ff.NewFlagSet(t.Name())
+		_ = fs.String('a', "apple", "", "foo string")
+		_ = fs.String('A', "apricot", "", "FOO string")
+		return fs
+	}
+	args := []string{"-a", "-A"}
+
+	t.Run("no env", func(t *testing.T) {
+		t.Parallel()
+		if err := ff.Parse(newFS(), args); err != nil {
+			t.Error(err)
+		}
+	})
+	t.Run("WithEnv", func(t *testing.T) {
+		t.Parallel()
+		if err := ff.Parse(
+			newFS(), args, ff.WithEnvVars(),
+		); !errors.Is(err, ff.ErrDuplicateFlag) {
+			t.Errorf("wanted ErrDuplicateFlag, got %+v", err)
+		}
+	})
+}