tread more carefully when loading device_auth and snmp_auth

lib/App/Netdisco/Util/DeviceAuth.pm

@@ -34,7 +34,13 @@ config changes over time. Returns a list which can replace C<device_auth>.
 =cut
 
 sub fixup_device_auth {
-  my $config = (setting('device_auth') || setting('snmp_auth'));
+  my $da = dclone setting('device_auth');
+  my $sa = dclone setting('snmp_auth');
+
+  die "error: both snmp_auth and device_auth are defined! use only device_auth.\n"
+    if scalar @$da and scalar @$sa;
+
+  my $config = ((scalar @$da) ? $da : $sa);
   my @new_stanzas = ();
 
   # new style snmp config