diff --git a/src/build.cc b/src/build.cc
index b392803..90e910f 100644
--- a/src/build.cc
+++ b/src/build.cc
@@ -97,6 +97,7 @@
 }
 
 void BuildStatus::BuildEdgeStarted(Edge* edge) {
+  assert(running_edges_.find(edge) == running_edges_.end());
   int start_time = (int)(GetTimeMillis() - start_time_millis_);
   running_edges_.insert(make_pair(edge, start_time));
   ++started_edges_;