[Ethereum] Error: Known Transaction

My program is relaying raw transactions to a geth node from a mobile client.

I noticed that a transaction with the identical content will yield an identical hash. e.g. calling the same function with the same parameters.

If I send this raw transaction more than once in quick succession (before there was time to mine it) I get the known transaction error.

How can I check if my transaction is known? Or put it another way: How can I compute the transaction hash from the raw transaction string?

In the code below, I am looking for a solution to someWayToComputeHashFrom

This is how I create my transaction string with eth-lightwallet. The contract object is a web3 contract.

getCertificateContract().then((contract) => {
            let payloadData = undefined;
            let functionName = undefined;
            functionName = 'registerAuditReport';
            payloadData = contract.registerAuditReport.getData(result.hash, documentId);

            let gasEstimate = w3.toHex(w3.eth.estimateGas({
                    to: contract.address,
                    data: payloadData
                }) + 10000);

            let nonce = w3.toHex(w3.eth.getTransactionCount(profile.address) + 10000);

            var rawTx = {
                nonce: nonce,
                gasPrice: gasPrice,
                gasLimit: gasEstimate,
                to: contract.address,
                from: profile.address,
                value: '0x00',
                data: payloadData
            };

            let rawTxString = txutils.functionTx(contract.abi, functionName, [result.hash, documentId], rawTx);

            let signedTxString = signing.signTx(wallet.keystore, wallet.pwDerivedKey, rawTxString, '0x' + senderAddress);


            let txHash = someWayToComputeHashFrom('0x' + rawTxHexString);
            w3.eth.sendRawTransaction('0x' + rawTxHexString, function (err, hash) {
                console.log('transaction hash is', hash);
            })
        })