We've noticed a few issues with clients using Universal Analytics this last month, when visits for the last day have been double the normal trend.
It then corrects itself a few hours later - so seems to be just a blip with the data processing at Google. Others have noticed the same problem.
The temporary fix is to only generate reports with time series ending the day before yesterday. i.e. ignore yesterday's data.
Now Google have officially acknowledged the problem
Looking forward to seeing that one fixed!