diff --git a/matrix/vm_eval.cc b/matrix/vm_eval.cc
index 7b2592819..8b9760ba3 100644
--- a/matrix/vm_eval.cc
+++ b/matrix/vm_eval.cc
@@ -323,9 +323,20 @@ ircd::m::vm::eval::operator()(const vector_view<m::event> &events)
 		// When a fault::EXISTS would not actually be revealed to the user in
 		// any way we can elide a lot of grief by checking this here first and
 		// skipping the event. The query path will be adequately cached anyway.
-		if(~(opts->warnlog | opts->errorlog) & fault::EXISTS)
-			if(event.event_id && m::exists(event.event_id))
+		if(event.event_id && ~(opts->warnlog | opts->errorlog) & fault::EXISTS)
+		{
+			// If the event is already being evaluated, wait here until the other
+			// evaluation is finished. If the other was successful, the exists()
+			// check will skip this, otherwise we have to try again here because
+			// this evaluator might be using different options/credentials.
+			sequence::dock.wait([&event]
+			{
+				return eval::count(event.event_id) == 0;
+			});
+
+			if(m::exists(event.event_id))
 				continue;
+		}
 
 		const auto status
 		{
diff --git a/matrix/vm_execute.cc b/matrix/vm_execute.cc
index a80241eb2..79ce174fa 100644
--- a/matrix/vm_execute.cc
+++ b/matrix/vm_execute.cc
@@ -435,7 +435,16 @@ ircd::m::vm::execute_pdu(eval &eval,
 			fault::GENERAL, "Internal room event denied from external source."
 		};
 
-	if(unlikely(eval::count(event_id) > 1))
+	// Wait for any pending duplicate evals before proceeding.
+	assert(eval::count(event_id));
+	sequence::dock.wait([&event_id]
+	{
+		return eval::count(event_id) <= 1;
+	});
+
+	// This branch won't be taken anymore with the above condition added; but
+	// leaving the code for reference right now.
+	if((false) && unlikely(eval::count(event_id) > 1))
 		throw error
 		{
 			fault::EXISTS, "Event is already being evaluated."