diff --git a/pungi/phases/init.py b/pungi/phases/init.py
index c347222f..19631447 100644
--- a/pungi/phases/init.py
+++ b/pungi/phases/init.py
@@ -14,17 +14,20 @@
 # along with this program; if not, see <https://gnu.org/licenses/>.
 
 
+import collections
+import glob
 import os
 import shutil
 
 from kobo.shortcuts import run
 
+from pungi import Modulemd
 from pungi.phases.base import PhaseBase
 from pungi.phases.gather import write_prepopulate_file
-from pungi.wrappers.createrepo import CreaterepoWrapper
-from pungi.wrappers.comps import CompsWrapper
-from pungi.wrappers.scm import get_file_from_scm, get_dir_from_scm
 from pungi.util import temp_dir
+from pungi.wrappers.comps import CompsWrapper
+from pungi.wrappers.createrepo import CreaterepoWrapper
+from pungi.wrappers.scm import get_dir_from_scm, get_file_from_scm
 
 
 class InitPhase(PhaseBase):
@@ -55,6 +58,9 @@ class InitPhase(PhaseBase):
         # download module defaults
         if self.compose.has_module_defaults:
             write_module_defaults(self.compose)
+            validate_module_defaults(
+                self.compose.paths.work.module_defaults_dir(create_dir=False)
+            )
 
         # write prepopulate file
         write_prepopulate_file(self.compose)
@@ -170,3 +176,30 @@ def write_module_defaults(compose):
         get_dir_from_scm(scm_dict, tmp_dir, logger=compose._logger)
         compose.log_debug("Writing module defaults")
         shutil.copytree(tmp_dir, compose.paths.work.module_defaults_dir(create_dir=False))
+
+
+def validate_module_defaults(path):
+    """Make sure there are no conflicting defaults. Each module name can only
+    have one default stream.
+
+    :param str path: directory with cloned module defaults
+    """
+    seen_defaults = collections.defaultdict(set)
+    for file in glob.glob(os.path.join(path, "*.yaml")):
+        for mmddef in Modulemd.objects_from_file(file):
+            if not isinstance(mmddef, Modulemd.Defaults):
+                continue
+            seen_defaults[mmddef.peek_module_name()].add(mmddef.peek_default_stream())
+
+    errors = []
+    for module_name, defaults in seen_defaults.items():
+        if len(defaults) > 1:
+            errors.append(
+                "Module %s has multiple defaults: %s"
+                % (module_name, ", ".join(sorted(defaults)))
+            )
+
+    if errors:
+        raise RuntimeError(
+            "There are duplicated module defaults:\n%s" % "\n".join(errors)
+        )
diff --git a/tests/test_initphase.py b/tests/test_initphase.py
index 5eaa3658..769e68fe 100644
--- a/tests/test_initphase.py
+++ b/tests/test_initphase.py
@@ -10,6 +10,7 @@ import sys
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
 
+from pungi import Modulemd
 from pungi.phases import init
 from tests.helpers import DummyCompose, PungiTestCase, touch
 
@@ -333,5 +334,66 @@ class TestWriteVariantComps(PungiTestCase):
         self.assertEqual(comps.write_comps.mock_calls, [])
 
 
+@unittest.skipUnless(Modulemd, "Skipped test, no module support.")
+class TestValidateModuleDefaults(PungiTestCase):
+
+    def _write_defaults(self, defs):
+        for mod_name, streams in defs.items():
+            for stream in streams:
+                mmddef = Modulemd.Defaults.new()
+                mmddef.set_version(1)
+                mmddef.set_module_name(mod_name)
+                mmddef.set_default_stream(stream)
+                mmddef.dump(
+                    os.path.join(self.topdir, "%s-%s.yaml" % (mod_name, stream))
+                )
+
+    def test_valid_files(self):
+        self._write_defaults({"httpd": ["1"], "python": ["3.6"]})
+
+        init.validate_module_defaults(self.topdir)
+
+    def test_duplicated_stream(self):
+        self._write_defaults({"httpd": ["1"], "python": ["3.6", "3.5"]})
+
+        with self.assertRaises(RuntimeError) as ctx:
+            init.validate_module_defaults(self.topdir)
+
+        self.assertIn(
+            "Module python has multiple defaults: 3.5, 3.6", str(ctx.exception)
+        )
+
+    def test_reports_all(self):
+        self._write_defaults({"httpd": ["1", "2"], "python": ["3.6", "3.5"]})
+
+        with self.assertRaises(RuntimeError) as ctx:
+            init.validate_module_defaults(self.topdir)
+
+        self.assertIn("Module httpd has multiple defaults: 1, 2", str(ctx.exception))
+        self.assertIn(
+            "Module python has multiple defaults: 3.5, 3.6", str(ctx.exception)
+        )
+
+    def test_handles_non_defaults_file(self):
+        self._write_defaults({"httpd": ["1"], "python": ["3.6"]})
+        touch(
+            os.path.join(self.topdir, "boom.yaml"),
+            "\n".join(
+                [
+                    "document: modulemd",
+                    "version: 2",
+                    "data:",
+                    "  summary: dummy module",
+                    "  description: dummy module",
+                    "  license:",
+                    "    module: [GPL]",
+                    "    content: [GPL]",
+                ]
+            ),
+        )
+
+        init.validate_module_defaults(self.topdir)
+
+
 if __name__ == "__main__":
     unittest.main()