diff --git a/src/porchlight/authn/routes.py b/src/porchlight/authn/routes.py
index 1ded1d2..08233d8 100644
--- a/src/porchlight/authn/routes.py
+++ b/src/porchlight/authn/routes.py
@@ -78,9 +78,13 @@ async def register_magic_link(request: Request, token: str) -> Response:
     if link is None:
         return HTMLResponse("<p>Invalid or expired registration link.</p>", status_code=400)
 
-    userid = await generate_unique_userid(user_repo)
-    user = User(userid=userid, username=link.username, groups=["users"])
-    await user_repo.create(user)
+    existing_user = await user_repo.get_by_username(link.username)
+    if existing_user is not None:
+        user = existing_user
+    else:
+        userid = await generate_unique_userid(user_repo)
+        user = User(userid=userid, username=link.username, groups=["users"])
+        await user_repo.create(user)
 
     await magic_link_service.mark_used(token)