diff --git a/src/poetry/repositories/pool.py b/src/poetry/repositories/pool.py
index 0e68cc6997b..4451385d4e6 100644
--- a/src/poetry/repositories/pool.py
+++ b/src/poetry/repositories/pool.py
@@ -62,6 +62,8 @@ def add_repository(
         Adds a repository to the pool.
         """
         repository_name = repository.name.lower()
+        if repository_name in self._lookup:
+            raise ValueError(f"{repository_name} already added")
 
         if default:
             if self.has_default():