The example saying:

This means that the compiler is allowed to optimize

  volatile int *p = somevalue;
  int s = *p + *p;

as if it said

  volatile int *p = somevalue;
  int tmp = *p;
  int s = tmp + tmp;

is incorrect. The clause about sequence points starts with "Futhermore, " so it is an additional restriction, and to boot it only talks about stores to the volatile-qualified object, not loads, so it is entirely inapplicable to this example.

The prior clause saying "Therefore any expression referring to such an object shall be evaluated strictly according to the rules of the abstract machine, as described in 5.1.2.3." applies, and the rules of the abstract machine for the binary + operator are that each operand is evaluated (in an unspecified order). So both dereferences must be evaluated.