depends on how many kittens there were, but if there were more than 6 total, not only would the smaller kittens have difficulty getting enough milk, it would be too much of a strain on Mom too.
And yes, theoretically this could happen again and again. Is there anything you can to do keep it from happening and getting the Mom indoors until she weans her kittens and getting her to a rescue group to be spayed and who would put all, including the kittens, up for adoption?
In fact, if that's not possible for you, a rescue group could take them now and foster them out.
Also, know that it is not unkinown for a mother kitten, if impregnated while she is nursing another litter, to actually kill the kittens in the first litter since a new Tom is now dominant, to care for the 2nd litter. Queens have also been known to kill kittens if there are simply too many for her to handle, and she would rather care for a lesser number completely than try to spread herself too thin and not be able to care for all adequately.
This would not a good situation to ponder about for many reasons.