你好，我的朋友

我叫本杰明
我住在伦敦
我两个月学习中文两个月了

朋友们, 你好，我的朋友 [Anglicized Chinese.  You are translating directly from, "How are you? My friends"

我叫本杰明, 我住在伦敦. 我两个月前开始学习中文了.

It is best to provide an English version.  This way you learn better and less guess work from us if you start to write something more complicated.

你好，我的朋友

我叫本杰明
我住在伦敦
我两个月学习中文两个月了

Here“两个月” is used as complement, decribing the time I have learnt Mandarin.

It always appears after an action.

你好，我的朋友们

我叫本杰明
我住在伦敦
我学了两个月中文了。

I think you got confused between the following 3 expressions.here i will give you examples:

1. Subject+Time+  verb（了）+noun.

     我       今天早上    学习了       中文。

2. Subject+Verb（了,optional）+noun+time of duration.

Expressing an action has been lasted for a period of time in the past.

我学习了两个月中文。I studied 2 month's Chinese.

3.Subject +Verb（了）+noun+time of duration+了.

Expressing an action has been lasted for a period of time,and it's still happening.

我学习了两个月中文了。I've been studying Chinese for 2 months already.