Remove duplicated object files in static libraries

When a static library link_whole to a bunch of other static libraries,
we have to extract all their objects recursively. But that could
introduce duplicated objects. ar is dumb enough to allow this without
error, but once the resulting static library is linked into an
executable or shared library, the linker will complain about duplicated
symbols.

mesonbuild/backend/backends.py

@@ -281,7 +281,8 @@ class Backend:
                                os.path.join('dummyprefixdir', fromdir))
 
     def flatten_object_list(self, target, proj_dir_to_build_root=''):
-        return self._flatten_object_list(target, target.get_objects(), proj_dir_to_build_root)
+        obj_list = self._flatten_object_list(target, target.get_objects(), proj_dir_to_build_root)
+        return list(dict.fromkeys(obj_list))
 
     def _flatten_object_list(self, target, objects, proj_dir_to_build_root):
         obj_list = []

test cases/unit/69 static link/lib/func17.c

@@ -0,0 +1,4 @@
+int func17()
+{
+  return 1;
+}

test cases/unit/69 static link/lib/func18.c

@@ -0,0 +1,6 @@
+int func17();
+
+int func18()
+{
+  return func17() + 1;
+}

test cases/unit/69 static link/lib/func19.c

@@ -0,0 +1,7 @@
+int func17();
+int func18();
+
+int func19()
+{
+  return func17() + func18();
+}

test cases/unit/69 static link/lib/meson.build

@@ -66,3 +66,15 @@ libfunc15 = static_library('func15', 'func15.c',
 libfunc16 = static_library('func16', 'func16.c',
   link_with : libfunc15,
   install : true)
+
+# Verify func17.c.o gets included only once into libfunc19, otherwise
+# func19-shared would failed with duplicated symbol.
+libfunc17 = static_library('func17', 'func17.c',
+  install : false)
+libfunc18 = static_library('func18', 'func18.c',
+  link_with : libfunc17,
+  install : false)
+libfunc19 = static_library('func19', 'func19.c',
+  link_whole : [libfunc17, libfunc18],
+  install : false)
+shared_library('func19-shared', link_whole : [libfunc19])