server: Detect duplicate client/bankd connection; drop new ones

we're dropping the current (new) connection as we don't really know which
is the "right" one. Dropping the new gives the old connection time to
timeout, or to continue to operate.  If we were to drop the old
connection, this could interrupt a perfectly working connection and opens
some kind of DoS.

Related: OS#5527
Change-Id: I00387dbc19d689415470e2f08df08a47a78b81c0
diff --git a/include/osmocom/rspro/ResultCode.h b/include/osmocom/rspro/ResultCode.h
index 1342990..9a2cbd2 100644
--- a/include/osmocom/rspro/ResultCode.h
+++ b/include/osmocom/rspro/ResultCode.h
@@ -25,6 +25,7 @@
 	ResultCode_illegalSlotId	= 3,
 	ResultCode_unsupportedProtocolVersion	= 4,
 	ResultCode_unknownSlotmap	= 5,
+	ResultCode_identityInUse	= 6,
 	ResultCode_cardNotPresent	= 100,
 	ResultCode_cardUnresponsive	= 101,
 	ResultCode_cardTransmissionError	= 102