Something definitely wrong somewhere.... I've never had that many bad starters, but certainly possible from the rebuilders these days. A lot of them may use "one" supplier for their parts including the pinion gears. Let us know what you finde.
One thing to make sure is that you check the whole circumference of the flywheel...
Try this from Auto Zone repair. Scroll down for GM starter shim checking.
http://www.autozone.com/az/cds/en_us...rInfoPages.htm