Even using the same callsign or flight number for outward AND inward flights is pretty silly. Why create the possibility of confusion by doubling up? Flight numbers don't cost money - so why not just give each flight its own number - it cannot be that difficult!
"Stubbing flights" (altering 1 of the 2 call signs) is a routine practice, meant to prevent the potential of just such incidents from occurring. Most times, the 2 aircraft would be operating in different Controllers' "Sectors" and on different frequencies, meaning no one would have been the wiser, but "Stubbing" is required nonetheless to eliminate any chance of this overlap occurrence. One was missed, and "could have been disastrous", as was stated. This incident highlights the criticality of the check and balance procedures that Commercial Airline Operations instill in their staff and their tools. A mistake was made, and that process will be corrected and retrained, I can assure you.
So it appears that in summary:
- old system assumptions limit the length of flight numbers
- in the USA, which has the highest density of flights and so hit the issue first, nobody insisted on solving it by extending field-length, and confusing/dangerous duplication is accepted
- outside the USA, workarounds have been found to avoid confusion/danger, and are enforced by convention.
Nothing going on, nothing to see here. Isn't it strange how many close calls there are this year alone?