SCM with multi streams

Scenario:

Currently we have source base as part of some source system, which has been built from many years. This has following folder structure.

Full Source code

-          Application layer

n  Application A

n  Application B

n  Application C

o    

o   …

-          Common Layer

o   Com M

o   Com N

o   Com O

o    

….

In the above common layer projects are common for App A, B, etc project.. Due to this inter dependencies we are thinking of making in to logical grouping of application layer and common layer project in to separate streams of RTC. With this common approach I am foreseeing 2 challenges:

1.       How do we ensure common code will not get duplicated in each stream.. for eg com M is applicable to App A, as well as B.. then where to push this

2.       How can we create common build which takes care of all application in to one final build

Or is it recommended to put this complete code in to one stream as different components??