diff --git a/gitflow-sh-setup b/gitflow-sh-setup
index 7fbef51..e1d5329 100755
--- a/gitflow-sh-setup
+++ b/gitflow-sh-setup
@@ -49,3 +49,51 @@
 	fi
 }
 
+#
+# gitflow_test_branches_equal()
+# 
+# Tests whether branches and their "origin" counterparts have diverged and need
+# merging first. It returns error codes to provide more detail, like so:
+# 
+# 0		Branch heads point to the same commit
+# 1		First given branch needs fast-forwarding
+# 2		Second given branch needs fast-forwarding
+# 3		Branch needs a real merge
+#
+gitflow_test_branches_equal() {
+	commit1=$(git rev-parse "$1")
+	commit2=$(git rev-parse "$2")
+	if [ "$commit1" != "$commit2" ]; then
+		base=$(git merge-base "$commit1" "$commit2")
+		short_base=$(git rev-parse --short "$base")
+
+		if [ "$commit1" = "$base" ]; then
+			return 1
+		elif [ "$commit2" = "$base" ]; then
+			return 2
+		else
+			return 3
+		fi
+	else
+		return 0
+	fi
+}
+
+gitflow_require_branches_equal() {
+	gitflow_require_local_branch "$1"
+	gitflow_require_remote_branch "$2"
+	gitflow_test_branches_equal "$1" "$2"
+	status=$?
+	if [ $status -gt 0 ]; then
+		warn "Branches '$1' and '$2' have diverged."
+
+		if [ $status -eq 1 ]; then
+			die "And branch '$1' may be fast-forwarded."
+		elif [ $status -eq 2 ]; then
+			die "And local branch '$1' is ahead of '$2'."
+		else
+			die "Branches need merging first."
+		fi
+	fi
+}
+