Per RFC 5280, section 4.2: "A certificate MUST NOT include more
than one instance of a particular extension". X509_verify_cert()
will fail on such a cert anyway. The below switches to using the
X509_get_ext_d2i() API correctly. Not sure the added log_debug()
calls are of much value; they won't do harm.

Index: ca.c
===================================================================
RCS file: /cvs/src/sbin/iked/ca.c,v
diff -u -p -r1.101 ca.c
--- ca.c	13 Feb 2024 12:25:11 -0000	1.101
+++ ca.c	25 Apr 2024 12:47:46 -0000
@@ -1985,13 +1985,13 @@ ca_x509_subjectaltname_do(X509 *cert, in
 	GENERAL_NAME *entry;
 	ASN1_STRING *cstr;
 	char idstr[IKED_ID_SIZE];
-	int idx, ret, i, type, len;
+	int crit, ret, i, type, len;
 	const uint8_t *data;
 
 	ret = -1;
-	idx = -1;
-	while ((stack = X509_get_ext_d2i(cert, NID_subject_alt_name,
-	    NULL, &idx)) != NULL) {
+	crit = -1;
+	if ((stack = X509_get_ext_d2i(cert, NID_subject_alt_name,
+	    &crit, NULL)) != NULL) {
 		for (i = 0; i < sk_GENERAL_NAME_num(stack); i++) {
 			entry = sk_GENERAL_NAME_value(stack, i);
 			switch (entry->type) {
@@ -2071,12 +2071,13 @@ ca_x509_subjectaltname_do(X509 *cert, in
 			}
 		}
 		sk_GENERAL_NAME_pop_free(stack, GENERAL_NAME_free);
-		if (ret != -1)
-			break;
-	}
-	if (idx == -1)
+	} else if (crit == -2)
+		log_debug("%s: multiple subjectAltName extensions", __func__);
+	else if (crit == -1)
 		log_debug("%s: did not find subjectAltName in certificate",
 		    __func__);
+	else
+		log_debug("%s: failed to decode subjectAltName", __func__);
 	return ret;
 }