It certainly is common for this to happen if ground rules are not established. The first thing I would recommend is having at least a simple signed contract between you and your hired help. Make sure that you both know what is and is not allowed with all images that are taken while on the clock, and add that to the contract.

Part of the reason that this is more common is that many second shooters may be inexperienced in the business aspects of event photography. They do not realize what holding a reputation as a business requires and all that goes into it. Many are looking to build a portfolio and experience at more or less your expense. This is in contrast to more experienced second shooters that mostly look at it as a favor to another professional, or mutual gain by sharing in the profits.

I would be concerned with what you said was happening with the images. If you post the images on your company page, then the second shooter does the same with no disclaimer, what happens if a bride sees both displays of the image? You both could be marked as untrustworthy just due to the confusion.

My experience is that it is most common to not allow any usage of the photos by the second shooter. This might seem like a poor deal for the second shooter just starting out and trying to build a portfolio, but it's just how it works! If they want to build a portfolio, second shooting might not be a great idea. They might be better off building a portfolio from actual weddings that they undertake. The second shooting experience might just be good to get the knowledge and less so to gain the images. They also could consider second shooting for free and be allowed to use images under an arrangement prior to the event.

One option to safeguard against this beyond a contract, is to provide your own memory cards to second shooters. That way at the end of the night, you collect the memory cards and know that the images are 1.safe with you 2.unable to be posted without authorization.