In file ipc_api.c, the function RPMessage_enqueMsg() handles incoming messages. In this function announcements are put into a queue (IpcUtils_Qput()) by the call to RPMessage_processAnnounceMsg. After this is handled the configured callback function is called.
After this, the message are copied to new buffers and inserted into a queue IpcUtils_Qput().The application callback function is called again.

//------------------------------------------{}RPMessage_enqueMsg(){}---------------------------------------------------//

#ifdef IPC_EXCLUDE_CTRL_TASKS
if (msg->dstAddr == IPC_CTRL_ENDPOINT_ID)
{
/* This message is for the ctrl endpoint */
RPMessage_Announcement amsg = (RPMessage_Announcement)msg->payload;
if (amsg->ctrl.type == CNTRLMSG_ANNOUNCE)
{
#if DEBUG_PRINT
SystemP_printf("RPMessage_enqueMsg ...CNTRLMSG_ANNOUNCE\n");
#endif
status = RPMessage_processAnnounceMsg(
(RPMessage_Announcement*)amsg, msg->srcProcId);
if(status != IPC_SOK)

{ SystemP_printf("RPMessage_processAnnounceMsg: Failed"); }

}
if ((status == IPC_SOK) && (NULL != gIpcObject.initPrms.newMsgFxn))
{
gIpcObject.initPrms.newMsgFxn(msg->srcAddr, msg->dstAddr, msg->srcProcId);
}
}
#endif /* IPC_EXCLUDE_CTRL_TASKS */
if (NULL != obj)
{
key = pOsalPrms->lockHIsrGate(module.gateSwi);

if (NULL != obj->recv_buffer)
{
memcpy(obj->recv_buffer, (void *)msg->payload, msg->dataLen);
obj->recv_buffer = NULL;
obj->payload.len = msg->dataLen;
obj->payload.src = msg->srcAddr;
obj->payload.procId = msg->srcProcId;

pOsalPrms->unLockHIsrGate(module.gateSwi, key);

if (NULL != gIpcObject.initPrms.newMsgFxn)

{ gIpcObject.initPrms.newMsgFxn(msg->srcAddr, msg->dstAddr, msg->srcProcId); }

if (NULL != pOsalPrms->unlockMutex)

{ pOsalPrms->unlockMutex(obj->semHandle); }

}
else
{
if( obj->endPt != msg->dstAddr)
{ SystemP_printf("WARNING: %d != %d\n", obj->endPt, msg->dstAddr); }

/* Allocate a buffer to copy the payload: */
size = msg->dataLen + sizeof(RPMessage_MsgElem);

/* HeapBuf_alloc() is non-blocking, so needs protection: HIsrGate has already been taken*/
payload = (RPMessage_MsgElem *)IpcUtils_HeapAlloc(&obj->heap, size, 0);

if (payload != NULL)
{
memcpy(payload->data, msg->payload, msg->dataLen);
payload->len = msg->dataLen;
payload->src = msg->srcAddr;
payload->procId = msg->srcProcId;

IpcUtils_Qput(&obj->queue, &payload->elem);
pOsalPrms->unLockHIsrGate(module.gateSwi, key);
if (NULL != gIpcObject.initPrms.newMsgFxn)
{
gIpcObject.initPrms.newMsgFxn(msg->srcAddr, msg->dstAddr, msg->srcProcId);
}

if (NULL != pOsalPrms->unlockMutex)
{ pOsalPrms->unlockMutex(obj->semHandle); }

}
else

{ pOsalPrms->unLockHIsrGate(module.gateSwi, key); SystemP_printf("IpcUtils_HeapAlloc failed: Failed to allocate buffer for payload. (%d %d %d %d %d)\n", msg->srcProcId, msg->srcAddr, msg->dstAddr, msg->dataLen, obj->endPt); status = IPC_EFAIL; }

}
}

//------------------------------------------{}RPMessage_enqueMsg(){}---------------------------------------------------//

The function RPMessage_enqueMsg first calls RPMessage_processAnnounceMsg (which enqueues the message inside it), then after returning from RPMessage_processAnnounceMsg it enqueues the message again, but in a different queue.

First queue is completely ignored , second queue is being created and used.