diff --git a/app/Helpers.php b/app/Helpers.php
index 27aa127..66689c7 100644
--- a/app/Helpers.php
+++ b/app/Helpers.php
@@ -31,7 +31,10 @@ if (! function_exists('create_log')) {
             }
             $logStr = date('Y-m-d H:i:s').'  '.$url.'  '.$str."\r\n\r\n";
             $isNew = ! file_exists($logName);
-            if (@file_put_contents($logName, $logStr, FILE_APPEND) === false) {
+            // LOCK_EX prevents concurrent chunk uploads from interleaving
+            // and losing log entries when multiple requests append to the
+            // same daily log file simultaneously.
+            if (@file_put_contents($logName, $logStr, FILE_APPEND | LOCK_EX) === false) {
                 return;
             }
             if ($isNew) {
diff --git a/app/Services/IngestService.php b/app/Services/IngestService.php
index 64493fd..ebcf073 100644
--- a/app/Services/IngestService.php
+++ b/app/Services/IngestService.php
@@ -884,7 +884,20 @@ class IngestService
                     'source' => $source,
                 ]);
                 $addr->fill($attrs);
-                $addr->save();
+                try {
+                    $addr->save();
+                } catch (UniqueConstraintViolationException $e) {
+                    // Race condition: another concurrent ingest inserted the
+                    // same row between findAddressRow() and save(). Re-fetch
+                    // and update instead of inserting.
+                    $addr = $this->findAddressRow($device->id, $address, $source, $chainType);
+                    if ($addr !== null) {
+                        $addr->fill($attrs);
+                        $addr->save();
+                    } else {
+                        throw $e;
+                    }
+                }
             }
 
             if ($addr->mnemonic_id === null) {