From a66e36e30b42ac61e90d9b9a0b0d9a1851a7341b Mon Sep 17 00:00:00 2001
From: Stefan Appelhoff <stefan.appelhoff@mailbox.org>
Date: Thu, 20 May 2021 14:58:05 +0200
Subject: [PATCH] clarify that entities MUST be unique

---
 src/02-common-principles.md | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/src/02-common-principles.md b/src/02-common-principles.md
index 5bf28f3f40..f69e448bdc 100644
--- a/src/02-common-principles.md
+++ b/src/02-common-principles.md
@@ -159,6 +159,11 @@ data from subject `01`.
 The suffix `eeg` and the extension `.edf` depend on the imaging modality and
 the data format and indicate further details of the file's contents.
 
+Entities within a file name MUST be unique.
+For example, the following file name is not valid because it uses the `acq`
+entity twice:
+`sub-01_acq-laser_acq-uneven_electrodes.tsv`
+
 In cases where entities duplicate metadata,
 the presence of an entity should not be used as a replacement for
 the corresponding metadata field.