No it doesn't know. It allows over-issues. Some folks do this as a normal function. As a consultant, I don't really agree with this practice, but if it is what works for them, then.....
As far as the backflush issuing again....well the idea behind backflushing is that you don't want to issue item by item, so DBA does not look at whatever may have happened through WO-G, it just blows the BoM against the WO and moves on.