diff --git a/spdxexp/extracts.go b/spdxexp/extracts.go
index 55afff3..923e6e9 100644
--- a/spdxexp/extracts.go
+++ b/spdxexp/extracts.go
@@ -1,5 +1,10 @@
 package spdxexp
 
+import (
+	"maps"
+	"slices"
+)
+
 // ExtractLicenses extracts licenses from the given expression without duplicates.
 // Returns an array of licenses or error if error occurs during processing.
 func ExtractLicenses(expression string) ([]string, error) {
@@ -8,14 +13,30 @@ func ExtractLicenses(expression string) ([]string, error) {
 		return nil, err
 	}
 
-	expanded := node.expand(true)
-	licenses := make([]string, 0)
-	allLicenses := flatten(expanded)
-	for _, licenseNode := range allLicenses {
-		licenses = append(licenses, *licenseNode.reconstructedLicenseString())
+	seen := map[string]struct{}{}
+	collectExtractedLicenses(node, seen)
+	return slices.Collect(maps.Keys(seen)), nil
+}
+
+func collectExtractedLicenses(n *node, seen map[string]struct{}) {
+	if n == nil {
+		return
 	}
 
-	licenses = removeDuplicateStrings(licenses)
+	if n.isExpression() {
+		collectExtractedLicenses(n.left(), seen)
+		collectExtractedLicenses(n.right(), seen)
+		return
+	}
 
-	return licenses, nil
+	reconstructed := n.reconstructedLicenseString()
+	if reconstructed == nil {
+		return
+	}
+
+	license := *reconstructed
+	if _, ok := seen[license]; ok {
+		return
+	}
+	seen[license] = struct{}{}
 }
diff --git a/spdxexp/extracts_test.go b/spdxexp/extracts_test.go
index f56676b..f7f7d7a 100644
--- a/spdxexp/extracts_test.go
+++ b/spdxexp/extracts_test.go
@@ -1,11 +1,49 @@
 package spdxexp
 
 import (
+	"context"
+	"os"
+	"os/exec"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 )
 
+const kernelHeadersLicense = `(GPL-2.0-only WITH Linux-syscall-note OR BSD-2-Clause) AND (GPL-2.0-only WITH Linux-syscall-note OR BSD-3-Clause) AND (GPL-2.0-only WITH Linux-syscall-note OR CDDL-1.0) AND (GPL-2.0-only WITH Linux-syscall-note OR Linux-OpenIB) AND (GPL-2.0-only WITH Linux-syscall-note OR MIT) AND (GPL-2.0-or-later WITH Linux-syscall-note OR BSD-3-Clause) AND (GPL-2.0-or-later WITH Linux-syscall-note OR MIT) AND Apache-2.0 AND BSD-2-Clause AND BSD-3-Clause AND BSD-3-Clause-Clear AND GFDL-1.1-no-invariants-or-later AND GPL-1.0-or-later AND (GPL-1.0-or-later OR BSD-3-Clause) AND GPL-1.0-or-later WITH Linux-syscall-note AND GPL-2.0-only AND (GPL-2.0-only OR Apache-2.0) AND (GPL-2.0-only OR BSD-2-Clause) AND (GPL-2.0-only OR BSD-3-Clause) AND (GPL-2.0-only OR CDDL-1.0) AND (GPL-2.0-only OR GFDL-1.1-no-invariants-or-later) AND (GPL-2.0-only OR GFDL-1.2-no-invariants-only) AND GPL-2.0-only WITH Linux-syscall-note AND GPL-2.0-or-later AND (GPL-2.0-or-later OR BSD-2-Clause) AND (GPL-2.0-or-later OR BSD-3-Clause) AND (GPL-2.0-or-later OR CC-BY-4.0) AND GPL-2.0-or-later WITH GCC-exception-2.0 AND GPL-2.0-or-later WITH Linux-syscall-note AND ISC AND LGPL-2.0-or-later AND (LGPL-2.0-or-later OR BSD-2-Clause) AND LGPL-2.0-or-later WITH Linux-syscall-note AND LGPL-2.1-only AND (LGPL-2.1-only OR BSD-2-Clause) AND LGPL-2.1-only WITH Linux-syscall-note AND LGPL-2.1-or-later AND LGPL-2.1-or-later WITH Linux-syscall-note AND (Linux-OpenIB OR GPL-2.0-only) AND (Linux-OpenIB OR GPL-2.0-only OR BSD-2-Clause) AND Linux-man-pages-copyleft AND MIT AND (MIT OR GPL-2.0-only) AND (MIT OR GPL-2.0-or-later) AND (MIT OR LGPL-2.1-only) AND (MPL-1.1 OR GPL-2.0-only) AND (X11 OR GPL-2.0-only) AND (X11 OR GPL-2.0-or-later) AND Zlib AND (copyleft-next-0.3.1 OR GPL-2.0-or-later)`
+
+var expectedKernelHeadersLicenses = []string{
+	"GPL-2.0-only WITH Linux-syscall-note",
+	"BSD-2-Clause",
+	"BSD-3-Clause",
+	"CDDL-1.0",
+	"Linux-OpenIB",
+	"MIT",
+	"GPL-2.0-or-later WITH Linux-syscall-note",
+	"Apache-2.0",
+	"BSD-3-Clause-Clear",
+	"GFDL-1.1-no-invariants-or-later",
+	"GPL-1.0-or-later",
+	"GPL-1.0-or-later WITH Linux-syscall-note",
+	"GPL-2.0-only",
+	"GFDL-1.2-no-invariants-only",
+	"GPL-2.0-or-later",
+	"CC-BY-4.0",
+	"GPL-2.0-or-later WITH GCC-exception-2.0",
+	"ISC",
+	"LGPL-2.0-or-later",
+	"LGPL-2.0-or-later WITH Linux-syscall-note",
+	"LGPL-2.1-only",
+	"LGPL-2.1-only WITH Linux-syscall-note",
+	"LGPL-2.1-or-later",
+	"LGPL-2.1-or-later WITH Linux-syscall-note",
+	"Linux-man-pages-copyleft",
+	"MPL-1.1",
+	"X11",
+	"Zlib",
+	"copyleft-next-0.3.1",
+}
+
 func TestExtractLicenses(t *testing.T) {
 	tests := []struct {
 		name              string
@@ -35,3 +73,32 @@ func TestExtractLicenses(t *testing.T) {
 		})
 	}
 }
+
+func TestExtractLicensesLicenseRefAndDedup(t *testing.T) {
+	licenses, err := ExtractLicenses("(LicenseRef-custom OR LicenseRef-custom) AND (DocumentRef-spdx-tool-1.2:LicenseRef-custom OR MIT)")
+	assert.NoError(t, err)
+	assert.ElementsMatch(t, []string{"LicenseRef-custom", "DocumentRef-spdx-tool-1.2:LicenseRef-custom", "MIT"}, licenses)
+}
+
+func TestExtractLicensesLongExpressionDoesNotHang(t *testing.T) {
+	if os.Getenv("GO_SPDX_EXTRACT_LICENSES_LONG_CHILD") == "1" {
+		licenses, err := ExtractLicenses(kernelHeadersLicense)
+		assert.NoError(t, err)
+		assert.ElementsMatch(t, expectedKernelHeadersLicenses, licenses)
+		return
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// #nosec G204 G702 -- safe in tests: re-executes current test binary with fixed arg
+	cmd := exec.CommandContext(ctx, os.Args[0], "-test.run", "^TestExtractLicensesLongExpressionDoesNotHang$")
+	cmd.Env = append(os.Environ(), "GO_SPDX_EXTRACT_LICENSES_LONG_CHILD=1")
+	output, err := cmd.CombinedOutput()
+	if ctx.Err() == context.DeadlineExceeded {
+		t.Fatalf("ExtractLicenses timed out on long expression: %s", output)
+	}
+	if err != nil {
+		t.Fatalf("child process failed: %v\n%s", err, output)
+	}
+}
diff --git a/spdxexp/helpers.go b/spdxexp/helpers.go
deleted file mode 100644
index 8ad73df..0000000
--- a/spdxexp/helpers.go
+++ /dev/null
@@ -1,24 +0,0 @@
-package spdxexp
-
-// flatten will take an array of nested array and return
-// all nested elements in an array. e.g. [[1,2,[3]],4] -> [1,2,3,4]
-func flatten[T any](lists [][]T) []T {
-	var res []T
-	for _, list := range lists {
-		res = append(res, list...)
-	}
-	return res
-}
-
-// removeDuplicateStrings will remove all duplicates from a slice
-func removeDuplicateStrings(sliceList []string) []string {
-	allKeys := make(map[string]bool)
-	list := []string{}
-	for _, item := range sliceList {
-		if _, value := allKeys[item]; !value {
-			allKeys[item] = true
-			list = append(list, item)
-		}
-	}
-	return list
-}