Allow duplicate modifiers on same function

[cgewecke]

Fixes a bug found running Synthetix where modifiers are invoked twice in the same function header, e.g

    function exchangeEtherForSNX()
        external
        payable
        rateNotInvalid(SNX)
        rateNotInvalid(ETH)
        notPaused
        returns (
            uint // Returns the number of SNX received
        )
    {
        return _exchangeEtherForSNX();
    }

PR disambiguates these using a counter.