Added explanation for "... has duplicates" error

Added a documentation page that explains the "... has duplicates"
error and what can be done to work around this problem.

Change-Id: I0346ad435a5f6cf7739268bd9076d66f4b119c3c
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
diff --git a/Documentation/error-messages.txt b/Documentation/error-messages.txt
index 795674b..d864081 100644
--- a/Documentation/error-messages.txt
+++ b/Documentation/error-messages.txt
@@ -12,6 +12,7 @@
 * link:error-branch-not-found.html[branch ... not found]
 * link:error-change-closed.html[change ... closed]
 * link:error-change-not-found.html[change ... not found]
+* link:error-has-duplicates.html[... has duplicates]
 * link:error-invalid-changeid.html[invalid Change-Id line format in commit message]
 * link:error-missing-changeid.html[missing Change-Id in commit message]
 * link:error-multiple-changeid-lines.html[multiple Change-Id lines in commit message]