diff --git a/src/lib/Hydra/Controller/Build.pm b/src/lib/Hydra/Controller/Build.pm
index 78e4b59a..4df80ddf 100644
--- a/src/lib/Hydra/Controller/Build.pm
+++ b/src/lib/Hydra/Controller/Build.pm
@@ -532,7 +532,7 @@ sub clone_submit : Chained('build') PathPart('clone/submit') Args(0) {
     my %currentBuilds;
     my $newBuild = checkBuild(
         $c->model('DB'), $build->project, $build->jobset,
-        $inputInfo, $nixExprInput, $job, \%currentBuilds, undef);
+        $inputInfo, $nixExprInput, $job, \%currentBuilds, undef, {});
 
     error($c, "This build has already been performed.") unless $newBuild;
     
diff --git a/src/lib/Hydra/Helper/AddBuilds.pm b/src/lib/Hydra/Helper/AddBuilds.pm
index e3115218..0fa3ed20 100644
--- a/src/lib/Hydra/Helper/AddBuilds.pm
+++ b/src/lib/Hydra/Helper/AddBuilds.pm
@@ -803,7 +803,7 @@ sub getPrevJobsetEval {
 
 # Check whether to add the build described by $buildInfo.
 sub checkBuild {
-    my ($db, $project, $jobset, $inputInfo, $nixExprInput, $buildInfo, $buildIds, $prevEval) = @_;
+    my ($db, $project, $jobset, $inputInfo, $nixExprInput, $buildInfo, $buildIds, $prevEval, $jobOutPathMap) = @_;
 
     my $jobName = $buildInfo->{jobName};
     my $drvPath = $buildInfo->{drvPath};
@@ -844,6 +844,14 @@ sub checkBuild {
                 return;
             }
         }
+
+        # Prevent multiple builds with the same (job, outPath) from
+        # being added.
+        my $prev = $$jobOutPathMap{$job->name . "\t" . $outPath};
+        if (defined $prev) {
+            print STDERR "    already scheduled as build ", $prev, "\n";
+            return;
+        }
         
         my $time = time();
         
@@ -886,6 +894,7 @@ sub checkBuild {
             });
 
         $buildIds->{$build->id} = 1;
+        $$jobOutPathMap{$job->name . "\t" . $outPath} = $build->id;
         
         if ($build->iscachedbuild) {
             print STDERR "    marked as cached build ", $build->id, "\n";
diff --git a/src/script/hydra-evaluator b/src/script/hydra-evaluator
index 33d1c8ce..ddf65f8b 100755
--- a/src/script/hydra-evaluator
+++ b/src/script/hydra-evaluator
@@ -124,6 +124,8 @@ sub checkJobset {
     my ($jobs, $nixExprInput) = evalJobs($inputInfo, $jobset->nixexprinput, $jobset->nixexprpath);
     my $evalStop = time;
 
+    my $jobOutPathMap = {};
+
     txn_do($db, sub {
         
         my $prevEval = getPrevJobsetEval($db, $jobset, 1);
@@ -138,7 +140,7 @@ sub checkJobset {
         foreach my $job (permute @{$jobs->{job}}) {
             next if $job->{jobName} eq "";
             print STDERR "  considering job " . $project->name, ":", $jobset->name, ":", $job->{jobName} . "\n";
-            checkBuild($db, $project, $jobset, $inputInfo, $nixExprInput, $job, \%buildIds, $prevEval);
+            checkBuild($db, $project, $jobset, $inputInfo, $nixExprInput, $job, \%buildIds, $prevEval, $jobOutPathMap);
         }
 
         # Update the last checked times and error messages for each