$ cat common-2.f
      program main
      common /foo/ a
      common /bar/ b
      equivalence (a,c)
      equivalence (b,c)
      c=3.
      print *,a
      print *,b
      end
$ gfortran common-2.f && ./a.out
   3.000000
   3.000000
$ gfortran -v
Reading specs from /home/zfkts/lib/gcc/ia64-unknown-linux-gnu/4.0.0/specs
Configured with: ../gcc-4.0-20041205/configure --prefix=/home/zfkts
--enable-languages=c,c++,f95 --disable-shared
Thread model: posix
gcc version 4.0.0 20041205 (experimental)
$ g77 common-2.f
common-2.f: In program `main':
common-2.f:5:
         equivalence (b,c)
                      ^
Attempt to EQUIVALENCE common areas `foo' and `bar' at (^)