pcrextend: suppress duplicate --bank= arguments

If the same bank is specified twice we'd extend the PCR twice in that bank in
one go, and write a userspace measurement log record that lists the same
digest algorithm twice. systemd-pcrlock refuses such records ("Failed to add
bank to event log record: File exists"), which in turn breaks everything that
reads the event log, for example systemd-report-sign-tpm2.

TEST-70-TPM2.pcrextend.sh deliberately calls --bank=sha256 --bank=sha256, and
on systems without a firmware event log (where TEST-70-TPM2.pcrlock.sh is
skipped and hence doesn't reset the log) TEST-70-TPM2.report-sign.sh then fails
because the TPM2 signer gets EEXIST from io.systemd.PCRLock.ReadEventLog.

src/pcrextend/pcrextend.c

@@ -176,6 +176,10 @@ static int parse_argv(int argc, char *argv[], char ***ret_args) {
                         return introspect_cli(SD_JSON_FORMAT_OFF);
                 }
 
+        /* Suppress duplicates: otherwise we'd extend the same bank twice, and write a measurement log
+         * record listing the same digest twice, which readers refuse. */
+        strv_sort_uniq(arg_banks);
+
         if (!!arg_file_system + arg_machine_id + arg_product_id + !!arg_login > 1)
                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL), "--file-system=, --machine-id, --product-id, --login= may not be combined.");