diff --git a/src/allmydata/scripts/magic_folder_cli.py b/src/allmydata/scripts/magic_folder_cli.py
index e25f7b541..245a9b6a6 100644
--- a/src/allmydata/scripts/magic_folder_cli.py
+++ b/src/allmydata/scripts/magic_folder_cli.py
@@ -153,6 +153,9 @@ def join(options):
     dmd_cap_file = os.path.join(options["node-directory"], u"private", u"magic_folder_dircap")
     collective_readcap_file = os.path.join(options["node-directory"], u"private", u"collective_dircap")
 
+    if os.path.exists(dmd_cap_file) or os.path.exists(collective_readcap_file):
+        raise usage.UsageError("Cannot join. Already joined.")
+
     fileutil.write(dmd_cap_file, dmd_write_cap)
     fileutil.write(collective_readcap_file, magic_readonly_cap)