duck.f90 contains a module called duck.
duck_prb.f90 wants to include duck.
This means that duck.f90 must be compiled before duck_prb.f90.

Example 1 (will fail):
  gfortran duck_prb.f90 duck.f90
/tmp/ccw36rNi.o: In function `MAIN__':
duck_prb.f90:(.text+0x22): undefined reference to `__duck_MOD_flink'
duck_prb.f90:(.text+0xa7): undefined reference to `__duck_MOD_grink'
collect2: ld returned 1 exit status

Example 2 (succeeds):
  gfortran duck.f90 duck_prb.f90
 Toop =           24
 Boop =           14

Example 3 (succeeds):
  gfortran -c duck.f90
  gfortran duck_prb.f90 duck.o
 Toop =           24
 Boop =           14

End of example.