Usually it means that more than one copy was distributed and you are waiting for the second copy to be validated and checked against yours. It is a normal procedure and you should not worry. If however, you are the only copy and you wait more that an hour or so, then there could be something 'wrong' with the validator, which will be fixed in due course,

Because it can take up to ten days for the second copy to be returned. Look at one of the WUs and you can see when the second copy has to be returned. If it has not been returned after that time then another copy is sent out which has to be returned in a shorter period, maybe 4 days. This will keep on until the second copy is returned, so theoretically you could be waiting weeks until a second copy is successfully returned.