buildPythonPackage: fail if two packages with the same name are in closure

pkgs/development/python-modules/generic/default.nix

@@ -103,6 +103,10 @@ python.stdenv.mkDerivation (builtins.removeAttrs attrs ["disabled"] // {
 
   postFixup = attrs.postFixup or ''
     wrapPythonPrograms
+
+    # check if we have two packagegs with the same name in closure and fail
+    # this shouldn't happen, something went wrong with dependencies specs
+    python ${./do_conflict.py}
   '';
 
   shellHook = attrs.shellHook or ''

pkgs/development/python-modules/generic/do_conflict.py

@@ -0,0 +1,29 @@
+import pkg_resources
+import collections
+import sys
+
+do_abort = False
+packages = collections.defaultdict(list)
+
+for f in sys.path:
+    for req in pkg_resources.find_distributions(f):
+        if req not in packages[req.project_name]:
+            if req.project_name == 'setuptools':
+                continue
+            packages[req.project_name].append(req)
+
+
+for name, duplicates in packages.iteritems():
+    if len(duplicates) > 1:
+        do_abort = True
+        print("Found duplicated packages in closure for dependency '{}': ".format(name))
+        for dup in duplicates:
+            print("  " + repr(dup))
+
+if do_abort:
+    print("")
+    print(
+        'Package duplicates found in closure, see above. Usually this '
+        'happens if two packages depend on different version '
+        'of the same dependency.')
+    sys.exit(1)