Missing check on registerOfferingFactory

[sangheraio]

There should be a check to prevent duplicate registration of an offeringFactory.

[satyamakgec]

I think we are covering this by the check
https://github.com/PolymathNetwork/polymath-core/blob/7413dbd27347b27c8923d762e8b17da7745cc9c3/contracts/Compliance.sol#L199-L200

Above check facilitate to put up the restriction on changing the offeringFactory reputation data. if the offering factory doesn't get proposed then it will be potentially opened for this duplicate registration and I think it doesn't affect our system. If we want to restrict this also then we need a mapping mapping(address => bool) registerdOfferingFactoryList to restrict any type of duplicate registrations.