Hmmm... Quite an interesting one this, because generally the DCS (check-in system) shouldn't allow this situation to occur. The system will only allocate a seat number if the number of boarding passes already issued at that point is less than the number of seats on the aircraft.

Ie: Total seats on board must be < Number of seat numbers already allocated.

This is quite self-explanatory because the system can't allocate a seat number if they've all been used up already!

The confusion (and double boarding passes) normally occurs when a check-in agent does a seat change. If pap1 changes from seat-a to seat-b, but is erroneously given his original boarding pass (for seat-a), then the system will show seat-b as taken and seat-a as free. This means that pap2 can take seat-a - and also receive a boarding pass for seat-a. Ergo: two passengers with a boarding pass for the same seat. A problem, but in the check-in system the "true" situation can be discovered.

So:

pap1: checks into 2a
** system prints out b/p for 2a **
pap1: asks to move to aisle
** system allocates 2c and notes 2a as free **
check-in agent: doesn't print out new b/p: and hands over b/p for 2a.
pap2: asks for window. System allocates 2a
check-in agent: prints out b/p for 2a

Result: 2 bp's for 2a.

A variation is as follows: normally, someone can only be changed from seat-a to seat-b if seat-b is showing as unoccupied. However, a supervisor can override this and can move you from a to b, even if b *is* occupied. In that situation though, the system will warn the supervisor that the person in seat b then needs to be told to get a new boarding pass because their seat has been changed.

Ie:

pap1 checks into 2c
pap2 checks into 3c
pap2 asks to change seats
pap2 is moved from 3c to 2c
** system warns: pap1 moved into 3c **
** pap1 is not given new b/p for 3c **
** pap2 is given correct b/p for 2c **

RESULT:

pap1 holds b/p for 2c
pap2 holds b/p for 2c
On-board: Confusion reigns

However, it doesn't sounds like this is what's happened here: and it's even more odd because the flight was full, staff had been onloaded (probably from standby), which means that DCS thought it hadn't assigned all the seats on the aircraft, and there was also some confusion over classes - so the seat-change into allocated seats seems to have involved cross-cabin changes too (a process known as exclusive seat changes).

So, given all of that, the only answer is that during the OLCI process, your seat number was not recorded in DCS. Probably due to a bug in the OLCI code or a comm's failure out to Amadeus in Germany.

THEORY:

Your friend: Checks in and is allocated seat 2C (say)
System: Cocks-up and gives you a BP but doesn't mark 2C as allocated
Other traveller: Checks in and is given 2C
Staff member: Onloaded into 30F

On-board:
Your friend is in 2C
Other traveller: Holds B/P for 2C

Confusion reigns until:

** C-crew and ground-crew realise they need a seat **
** Kick off staff in 30F **

Other passenger: Resigns himself to 30F.


And if that hasn't suitably muddied the waters, I don't know what would!

P.S. Get your B/P at the airport from the SSCI machines ;-)