typo: Double word "of" (#2367)

Author: nschonni

locale/en/blog/vulnerability/september-2016-security-releases.md

@@ -138,7 +138,7 @@ As mitigation, OpenSSL has moved DES-based ciphers from the `HIGH` to `MEDIUM` g
 
 An overflow can occur in `MDC2_Update()` under certain circumstances resulting in an out of bounds (OOB) error. This attack is impractical on most platforms due to the size of data required to trigger the OOB error.
 
-Node.js is impacted by this flaw but due to the impracticalities of exploiting it and the very low usage of of MDC-2, it is _very low_ severity for Node.js users.
+Node.js is impacted by this flaw but due to the impracticalities of exploiting it and the very low usage of MDC-2, it is _very low_ severity for Node.js users.
 
 **Assessment**: All versions of Node.js are **affected** by this vulnerability.