Left Hand Signal In Left Hand Lane

i found on this site that i need to signal if it affect traffic . but what is the rule when turning left in a left hand land with a car that is behind you turning left as well if you are both in the left hand turn lane

It doesn't matter where you are or what you're doing, the law is the same: IF traffic can be affected, you must signal. Whether traffic could be affected in the situation you described is a question of fact and it depends on whether the officer charging in that situation could articulate how traffic was affected.

Oncoming traffic would be affected. The oncoming drivers may not see/recognize that there is a designated left turn lane there, particularly at night.