I've actually read in many running references that up to daTE research says it's actually better. It means your metabolism gets a double boost. Also the same gains are supposed to be reported.

I don't recall whether or not they studied gains of endurance versus speed / calorie expenditure. I suspect endurance would not improve to the same extent. However some ultra runners do two runs in a day to get their kms up - i guess the gap between runs allows some initial recovery and refueling enough to do the extra kms that might be too stressfuL on a regular basis within one day otherwise.

Best test - give it a go and see how it works for you