diff --git a/build-flow.sh b/build-flow.sh
new file mode 100644
index 0000000..3f3c97f
--- /dev/null
+++ b/build-flow.sh
@@ -0,0 +1,2 @@
+#!/bin/bash
+find ./src -name '*.js' -not -path '*/__*' | while read filepath; do cp $filepath `echo $filepath | sed 's/\\/src\\//\\/lib\\//g'`.flow; done
\ No newline at end of file
diff --git a/build-ts.sh b/build-ts.sh
new file mode 100644
index 0000000..94d98b8
--- /dev/null
+++ b/build-ts.sh
@@ -0,0 +1,2 @@
+#!/bin/bash
+find ./src -name '*.d.ts' -not -path '*/__*' | while read filepath; do cp $filepath `echo $filepath | sed 's/\\/src\\//\\/lib\\//g'`; done
\ No newline at end of file
diff --git a/package.json b/package.json
index ddb6a08..b193f32 100644
--- a/package.json
+++ b/package.json
@@ -64,10 +64,12 @@
     ]
   },
   "scripts": {
-    "build": "npm run build-cjs && npm run build-flow && npm run build-ts",
+    "build": "npm run build-cjs && npm run build-flow-sh && npm run build-ts-sh",
     "build-cjs": "rimraf lib && babel src --ignore __tests__,__mocks__ -d lib",
     "build-flow": "find ./src -name '*.js' -not -path '*/__*' | while read filepath; do cp $filepath `echo $filepath | sed 's/\\/src\\//\\/lib\\//g'`.flow; done",
+    "build-flow-sh": "sh build-flow.sh",
     "build-ts": "find ./src -name '*.d.ts' -not -path '*/__*' | while read filepath; do cp $filepath `echo $filepath | sed 's/\\/src\\//\\/lib\\//g'`; done",
+    "build-ts-sh": "sh build-ts.sh",
     "watch": "jest --watch",
     "coverage": "jest --coverage --maxWorkers 2",
     "lint": "npm run eslint && npm run tslint && npm run tscheck",
diff --git a/src/index.js b/src/index.js
index 3aa9f49..c3cb70e 100755
--- a/src/index.js
+++ b/src/index.js
@@ -114,7 +114,22 @@ async function createCounterIfNotExist(
       count: settings.startAt - settings.incrementBy,
     }): any);
 
-    await existedCounter.save();
+    try {
+      // this might fail if invoked in parallel
+      await existedCounter.save();
+    } catch (e) {
+      if (isMongoDuplicateError(e)) {
+        // just to be consistent with the method return type
+        // but to tell the truth it could return boolean at this point
+        existedCounter = (await IC.findOne({
+          model: settings.model,
+          field: settings.field,
+          groupingField,
+        }).exec(): any);
+      } else {
+        throw e; // other unhandled errors
+      }
+    }
   }
 
   return (existedCounter: any);
@@ -177,7 +192,7 @@ async function preSave(
       doc.set(settings.field, count);
 
       // $FlowFixMe
-      doc.__maiRanOnce = true; // eslint-disable-line
+      doc.__maiRanOnce = true; // eslint-disable-line
     }
 
     next();