A foreign resident of Japan unwittingly lived under a fake identity for six months until it came to light that he had been mixed up with an uncannily similar stranger.

The man in his 20s went to register his residency status at Tokushima City Hall in November, an official from the municipality said on Tuesday.

But in a bureaucratic blunder, he was assigned the identity of a completely different person — who had the same name, same nationality and even the same birthday.

When the man came to city hall, staff searched the national ID number registry and found that someone with the same details lived in another city, the official said.

After exchanges with the man, who spoke minimal Japanese, the staff mistakenly understood that he was moving from one city to another, and attached the existing residency details to him.

Half a year later, the national pension service noticed that something was wrong, and informed the city of their error.

"We have learned that there is still room for error" even when proper steps are taken, the Tokushima official said, declining to identify either of the doppelgangers-on-paper.

The other person had already left Japan, meaning the mistake did not come to light sooner, he added.

Japan launched an ID number system called "My Number" in 2015, but some people have said the new system risks human errors and data leaks.