Handle same Framework multiple times in Xcode.

mesonbuild/backend/xcodebackend.py

diff --git a/mesonbuild/backend/xcodebackend.py b/mesonbuild/backend/xcodebackend.py
index a5c6931..46f3b14 100644
--- a/mesonbuild/backend/xcodebackend.py
+++ b/mesonbuild/backend/xcodebackend.py
@@ -131,8 +131,8 @@
         self.items = []
 
     def add_item(self, key, value, comment=''):
-        item = PbxDictItem(key, value, comment)
         assert key not in self.keys
+        item = PbxDictItem(key, value, comment)
         self.keys.add(key)
         self.items.append(item)
 
@@ -601,7 +601,9 @@
                 if isinstance(dep, dependencies.AppleFrameworks):
                     for f in dep.frameworks:
                         fw_dict = PbxDict()
-                        objects_dict.add_item(self.native_frameworks[f], fw_dict, f'{f}.framework in Frameworks')
+                        fwkey = self.native_frameworks[f]
+                        if fwkey not in objects_dict.keys:
+                            objects_dict.add_item(fwkey, fw_dict, f'{f}.framework in Frameworks')
                         fw_dict.add_item('isa', 'PBXBuildFile')
                         fw_dict.add_item('fileRef', self.native_frameworks_fileref[f], f)