Fix logging of MID matching.

Recreated request from observation stores may not have a valid MID.

Signed-off-by: Achim Kraus <achim.kraus@bosch-si.com>

californium-core/src/main/java/org/eclipse/californium/core/network/UdpMatcher.java

@@ -255,7 +255,7 @@ public Exchange receiveResponse(final Response response) {
 		EndpointContext context = exchange.getEndpointContext();
 		Request currentRequest = exchange.getCurrentRequest();
 		int requestMid = currentRequest.getMID();
-		if (context == null || requestMid == Message.NONE) {
+		if (context == null) {
 			LOGGER.debug("ignoring response {}, request pending to sent!", response);
 			return null;
 		}
@@ -264,8 +264,16 @@ public Exchange receiveResponse(final Response response) {
 			if (endpointContextMatcher.isResponseRelatedToRequest(context, response.getSourceContext())) {
 				if (response.getType() == Type.ACK && requestMid != response.getMID()) {
 					// The token matches but not the MID.
-					LOGGER.warn("possible MID reuse before lifetime end for token {}, expected MID {} but received {}",
-							response.getTokenString(), requestMid, response.getMID());
+					if (requestMid != Message.NONE) {
+						LOGGER.debug(
+								"possible MID reuse before lifetime end for token {}, expected MID {} but received {}",
+								response.getTokenString(), requestMid, response.getMID());
+					} else if (exchange.isNotification()) {
+						LOGGER.debug("possible duplicated ACK [MID={}] for observe [token={}]", response.getMID(),
+								response.getTokenString());
+					} else {
+						LOGGER.error("response {} matched to request without MID {}", response, currentRequest);
+					}
 					// when nested blockwise request/responses occurs (e.g.
 					// caused by retransmission), a old response may stop the
 					// retransmission of the current blockwise request. This