playBunny: I agree with Thad that the number generator itself is probably okay and that it's the use that's at fault.

I think that the problem is only on the opening rolls. I know that the checking of pairs of dice within games hasn't been done yet but I suspect that a high occurence of duplication, such as there is for the opening rolls, would have been noticed much sooner than now and by many more people.

I know that I'd occasionally notice that the opponent's opening dice came out the same as mine, or vice versa, but I never went beyond that, to seeing it as a pattern. If it were happening throughout the game then I'm sure that I would have noticed and other, more observent people, would have seen it sooner.

So, assuming that it is an opening rolls issue, we must be looking for code that is special to the start of a game. One obvious contender is the rolling of dice for who goes first. In real backgammon, each player rolls a dice and the one with the higher value gets both to play with. After that first move the two players pick up their individual dice and thereafter take care of their own rolls.

If I were coding a backgammon server then I wouldn't bother with that. I'd simply toss a binary digit to see who was to start and then roll the starting player's dice using the same code as every other roll. However, if I were to code a simulation of the real live start action then there'd be the opportunity for error.

What might happen then is that I use one dice from each player for the first player's roll but then re-use one of those dice for the second player, presumably the dice that they rolled to see who started.

This, if Fencer is doing such a simulation, is the prime suspect for the bug. If you look at the example matches below then you can see clearly that there's at least one common dice in the majority of games. It's somthing that should be frequent (55% - the same odds as getting one man off the bar into a home table with 2 points open) but not that frequent.

If one of the dice is always the same and the other is a fair roll then duplication of both would occur with a frequency of 1/6 rather than the 1/18 that's expected, so 3 times more often.

The interesting thing about this bug is that there are exceptions. Although there are none in wetware's matches below, there are a few in mine and more in Resher's. That must be caused by something. One theory is that perhaps when the starting player swaps the dice before moving this somehow breaks the connection between the forst and second players' dice. I can't think why that should be the case and haven't played any matches that I can test the theory with.