Re: How often did Duncan guard Shaq?
When Robinson was there, he often guarded Shaq, and even 6'6" Malik Rose at times. But Duncan guarded Shaq at times, certainly not frequently enough that you could use head to head stats for a comparison.
When both Duncan and Robinson were younger and each very important to the offense, I know the strategy at times was to alternate them on Shaq depending on who picks up a foul. But when Robinson's role was reduced in '01, and especially his last 2 seasons in '02 and '03, he guarded Shaq much more frequently. But even in their '99 playoff series, I remember Robinson defending Shaq most of the time. Once Shaq went to Phoenix, he usually matched up with Duncan when both were on the court, that's because Duncan was almost always playing center by this point, and Shaq was traded to Phoenix largely to match up with Duncan.
But before that, we have to remember that Duncan played most of his first few years with a center in Robinson. Then his last 2-3 years he was pretty much splitting time between playing with a center(Robinson), and a power forward(Malik Rose). But then the Spurs added Rasho and Horry as well as eventually Mohammed so again, he was splitting time between playing with a center and power forward even during the post Robinson era from '04-'06. From '07 on, Duncan has spent the vast majority of his playing time with a center so as a result, he's ended up matching up with Shaq much more. Unfortunately, Shaq was old by this point.
There is a game I uploaded on youtube where Shaq and Duncan went head to head for most of the game in their primes on Christmas in the 1999-2000 season. Shaq won the match up decisively shutting down Duncan and still getting his 30+, but Duncan made him work for his points.
They also guarded each other quite a bit in the 2002 WCSF because Robinson was injured most of the series, but they didn't guard each other that much in those games until the latter part of the game. However they did guard each other quite well. Both had a difficult time scoring. Shaq because he was dealing with several injuries, Duncan because he didn't have enough help because that was his worst cast of his career and because Robinson was out. And also because both were good post defenders and still saw quite a few double teams.
But yeah, you can't make a statistical comparison unless you go back and watch every game, and track what each player did when they were matched up with each other.
This is a big problem when people try to use head to head stats. I've seen plenty of games where the stars didn't guard each other much. I've often seen Hakeem/Robinson stats cited, but I remember games where Robinson was guarded more by Thorpe and Hakeem was guarded more by Rodman and even JR Reid. Same thing goes for Ewing vs Robinson, Shaq vs Hakeem ect.