sceNetAdhocMatching: Don't list a member twice

A child the host accepts right after another one learns about that one
twice: from the sibling list in its accept packet, and from the BIRTH
packet the host sends about it. It added it both times, so with three or
four players the last one to join had an extra member (pspautotests
net/adhoc/group/matching, not recorded on PSPs yet).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Core/HLE/sceNetAdhocMatching.cpp

@@ -1024,6 +1024,11 @@ void actOnBirthPacket(SceNetAdhocMatchingContext * context, SceNetEtherAddr * se
 	SceNetEtherAddr mac;
 	memcpy(&mac, context->rxbuf + 1, sizeof(SceNetEtherAddr));
 
+	// A child accepted just after another already has it from its accept packet's sibling list.
+	if (findPeer(context, &mac) != NULL || isMacMatch(&mac, &context->mac)) {
+		return;
+	}
+
 	// Allocate Memory
 	SceNetAdhocMatchingMemberInternal * sibling = (SceNetAdhocMatchingMemberInternal *)malloc(sizeof(SceNetAdhocMatchingMemberInternal));