Both incidents involving the 47,000-dwt Garnet Express (built 2013) took place at the Belawan anchorage.
The first illegal boarding took place at the very end of July when three robbers boarded the ship while she was at anchor.
The crew spotted the robbers and the alarm was raised. Upon hearing the alarm, the robbers escaped immediately but stole tank cleaning equipment.
It has now emerged that the Marshall Islands flagged ship was also the target of robbers on 4 August just before 9am local time.
On this occasion the tanker was alongside when two robbers boarded the ship from a small wooden boat.
When the crew spotted the robbers, they escaped with several pieces of corrosion proof zinc plates that were attached at the tanker astern.
Shipowners operating in Indonesia face one of the highest chances of a piracy attack in the world where around one-third of sea crimes occur.
Almost all attacks are opportunistic ‘smash and grab’ type robberies on vessels in port or at anchor, according to UK-based security consultants AKE Maritime.
Details of the latest attack feature in the August piracy report of the Regional Cooperation Agreement on Combating Piracy and Armed Robbery against Ships in Asia (ReCAAP).
The attack on the Garnet Express was one of twelve such incidents in Asia last month according to ReCAAP.
Almost a year ago K Line's 1,708-teu containership Pearl River Bridge (built 2008) was subjected to a double robbery at the very same port in Indonesia.
