diff --git a/http3/headers.go b/http3/headers.go
index edad2c80..e9f2df70 100644
--- a/http3/headers.go
+++ b/http3/headers.go
@@ -30,7 +30,7 @@ type header struct {
 
 func parseHeaders(headers []qpack.HeaderField, isRequest bool) (header, error) {
 	hdr := header{Headers: make(http.Header, len(headers))}
-	var readFirstRegularHeader bool
+	var readFirstRegularHeader, readContentLength bool
 	var contentLengthStr string
 	for _, h := range headers {
 		// field names need to be lowercase, see section 4.2 of RFC 9114
@@ -74,7 +74,14 @@ func parseHeaders(headers []qpack.HeaderField, isRequest bool) (header, error) {
 			readFirstRegularHeader = true
 			switch h.Name {
 			case "content-length":
-				contentLengthStr = h.Value
+				// Ignore duplicate Content-Length headers.
+				// Fail if the duplicates differ.
+				if !readContentLength {
+					readContentLength = true
+					contentLengthStr = h.Value
+				} else if contentLengthStr != h.Value {
+					return header{}, fmt.Errorf("contradicting content lengths (%s and %s)", contentLengthStr, h.Value)
+				}
 			default:
 				hdr.Headers.Add(h.Name, h.Value)
 			}
diff --git a/http3/headers_test.go b/http3/headers_test.go
index ac09e932..7fcf675d 100644
--- a/http3/headers_test.go
+++ b/http3/headers_test.go
@@ -89,6 +89,32 @@ var _ = Describe("Request", func() {
 		Expect(err.Error()).To(ContainSubstring("invalid content length"))
 	})
 
+	It("rejects multiple Content-Length headers, if they differ", func() {
+		headers := []qpack.HeaderField{
+			{Name: ":path", Value: "/foo"},
+			{Name: ":authority", Value: "quic.clemente.io"},
+			{Name: ":method", Value: "GET"},
+			{Name: "content-length", Value: "42"},
+			{Name: "content-length", Value: "1337"},
+		}
+		_, err := requestFromHeaders(headers)
+		Expect(err).To(MatchError("contradicting content lengths (42 and 1337)"))
+	})
+
+	It("deduplicates multiple Content-Length headers, if they're the same", func() {
+		headers := []qpack.HeaderField{
+			{Name: ":path", Value: "/foo"},
+			{Name: ":authority", Value: "quic.clemente.io"},
+			{Name: ":method", Value: "GET"},
+			{Name: "content-length", Value: "42"},
+			{Name: "content-length", Value: "42"},
+		}
+		req, err := requestFromHeaders(headers)
+		Expect(err).ToNot(HaveOccurred())
+		Expect(req.ContentLength).To(Equal(int64(42)))
+		Expect(req.Header.Get("Content-Length")).To(Equal("42"))
+	})
+
 	It("rejects pseudo header fields defined for responses", func() {
 		headers := []qpack.HeaderField{
 			{Name: ":path", Value: "/foo"},