patman: Avoid duplicate sign-offs

Keep track of all Signed-off-by tags in a commit and silently suppress any
duplicates.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/README b/tools/patman/README
index b3aba13..5fb508b 100644
--- a/tools/patman/README
+++ b/tools/patman/README
@@ -192,6 +192,7 @@
 	A sign-off is added automatically to your patches (this is
 	probably a bug). If you put this tag in your patches, it will
 	override the default signoff that patman automatically adds.
+	Multiple duplicate signoffs will be removed.
 
  Tested-by: Their Name <email>
  Reviewed-by: Their Name <email>