Stratego:
--------------------------------------------------
module foo
imports liblib
strategies

  main =
    iset-isect
--------------------------------------------------

Compile:
--------------------------------------------------
martin@linux:~/tmp/bug-strc> strc -i foo.str
[ strc | info ] Compiling 'foo.str'
[ strc | info ] Front-end succeeded : [user/system] = [3.62s/0.68s]
[ strc | info ] Optimization succeeded -O 2 : [user/system] = [0.16s/0.07s]
[ strc | info ] Back-end succeeded : [user/system] = [0.17s/0.12s]
[ strc | error ] Command failed :
foo.c: In function `main_0_0':
foo.c:17: error: `iset' undeclared (first use in this function)
foo.c:17: error: (Each undeclared identifier is reported only once
foo.c:17: error: for each function it appears in.)
--------------------------------------------------

C:
--------------------------------------------------
ATerm main_0_0 (ATerm t)
{
t = iset-isect(t);
--------------------------------------------------

liblib.str:
--------------------------------------------------
module liblib
imports
  libstrategolib
  libxtclib
--------------------------------------------------

New try:
--------------------------------------------------
module foo
imports libstrategolib
strategies

  main =
    iset-isect
--------------------------------------------------

Compile:
--------------------------------------------------
martin@linux:~/tmp/bug-strc> strc -i foo.str
[ strc | info ] Compiling 'foo.str'
[ strc | info ] Front-end succeeded : [user/system] = [2.20s/0.59s]
[ strc | info ] Optimization succeeded -O 2 : [user/system] = [0.12s/0.07s]
[ strc | info ] Back-end succeeded : [user/system] = [0.16s/0.12s]
[ strc | error ] Command failed :
foo.c: In function `main_0_0':
foo.c:18: error: too few arguments to function `iset_isect_0_1'
--------------------------------------------------

That's right.

New try:
--------------------------------------------------
module foo
strategies

  main =
    iset-isect

  external iset-isect(|t : ATerm())
  external iset-isect(|t : ATerm())
--------------------------------------------------

Compile:
--------------------------------------------------
martin@linux:~/tmp/bug-strc> strc -i foo.str
[ strc | info ] Compiling 'foo.str'
[ strc | info ] Front-end succeeded : [user/system] = [0.30s/0.48s]
[ strc | info ] Optimization succeeded -O 2 : [user/system] = [0.01s/0.05s]
[ strc | info ] Back-end succeeded : [user/system] = [0.15s/0.14s]
[ strc | error ] Command failed :
foo.c: In function `main_0_0':
foo.c:17: error: `iset' undeclared (first use in this function)
foo.c:17: error: (Each undeclared identifier is reported only once
foo.c:17: error: for each function it appears in.)
--------------------------------------------------

That's wrong. Conclusion: in this case, strc cannot handle duplicate definitions of external symbols.