diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -18,6 +18,10 @@
 the change.  GitHub pull requests are preferred for relatively straightforward
 changes where the contributor already has a GitHub account.
 
+A change should be submitted by only one method.  For example, please do not
+open a GitHub pull request and create a Phabricator review for the same change
+(unless explicitly requested to do so by a FreeBSD committer).
+
 ## GitHub Pull Requests
 
 Presently, GitHub 'freebsd-src' repository is one of the publish-only services