Females can, and will lay infertile eggs from time to time. Infertile means they are not fertile cause a male was not present to fertilize them. If NO male was present just toss them away.. NOW are you 100% positive that the dragon you added to the cage was in fact female??
Housing females together can be done. Some females can be territorial just like the males, so watch for one female dominating the other. If that's the case then they must be housed alone. It's always best to keep dragons alone. Males will fight each other, and constantly breed females if kept together, so I always suggest to house males alone always, and just introduce to females for the purpose of breeding only. I have been lucky keeping my females housed together though. Just got to make sure they are compatable (some just aren't)
Before putting new dragons into an existing dragons cage you should really quarantine for a period a time, and get their fecal checked for parasites. In this quarantine period you want to make sure they are not going to introduce parasites, and disease to each other
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