BIG FAT CONFUSION I had a telephone interview from two different companies,in different day (Please see below)?

I applied for an administrative role in several companies, I was interviewed on the phone by one of these, and on the following day I received another call with the same job description but from a different company. Because I thought it was the same company I told them I was already interviewed, however I just realised that I misunderstood the situation. How do I go about it? I still want to do the telephone interview...how should I explaine the circumstance? or rather should I explaine it?